feat(gui): M1 shared core — extract src/gui/ and replace -Dgui with -Drenderer

Move shared types out of src/win32/ into a new src/gui/ package:
- xy.zig: generic XY(T) coordinate type
- xterm.zig: 256-color palette table (used by all GPU renderers)
- GlyphIndexCache.zig: glyph→atlas-slot mapping
- Cell.zig: Rgba8 and Cell extracted from d3d11.zig
- GlyphRasterizer.zig: comptime interface spec/checker

Update src/win32/ imports to point to ../gui/ and delete the
moved originals.

Replace the -Dgui bool option with -Drenderer enum { terminal, gui,
d3d11 }, wiring .d3d11 to the existing win32 DirectWrite path and
keeping build_options.gui = (renderer != .terminal) for tui.zig.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

build.zig

@@ -3,6 +3,8 @@ const builtin = @import("builtin");
 
 const optimize_deps = .ReleaseFast;
 
+pub const Renderer = enum { terminal, gui, d3d11 };
+
 pub fn build(b: *std.Build) void {
     const all_targets = b.option(bool, "all_targets", "Build all known good targets during release builds (default: no)") orelse false;
     const tracy_enabled = b.option(bool, "enable_tracy", "Enable tracy client library (default: no)") orelse false;
@@ -10,7 +12,7 @@ pub fn build(b: *std.Build) void {
     const strip = b.option(bool, "strip", "Disable debug information (default: no)");
     const use_llvm = b.option(bool, "use_llvm", "Enable llvm backend (default: none)");
     const pie = b.option(bool, "pie", "Produce an executable with position independent code (default: none)");
-    const gui = b.option(bool, "gui", "Standalone GUI mode") orelse false;
+    const renderer = b.option(Renderer, "renderer", "Renderer backend: terminal (TUI, default), d3d11 (GPU on Windows via DirectWrite), gui (GPU on Linux/macOS/Windows via wio+sokol_gfx)") orelse .terminal;
     const test_filters = b.option([]const []const u8, "test-filter", "Skip tests that do not match any filter") orelse &[0][]const u8{};
 
     const run_step = b.step("run", "Run the app");
@@ -47,7 +49,7 @@ pub fn build(b: *std.Build) void {
         strip,
         use_llvm,
         pie,
-        gui,
+        renderer,
         version.items,
         all_targets,
         test_filters,
@@ -65,7 +67,7 @@ fn build_development(
     strip: ?bool,
     use_llvm: ?bool,
     pie: ?bool,
-    gui: bool,
+    renderer: Renderer,
     version: []const u8,
     _: bool, // all_targets
     test_filters: []const []const u8,
@@ -87,7 +89,7 @@ fn build_development(
         strip orelse false,
         use_llvm,
         pie,
-        gui,
+        renderer,
         version,
         test_filters,
     );
@@ -104,7 +106,7 @@ fn build_release(
     _: ?bool, //release builds control strip
     use_llvm: ?bool,
     pie: ?bool,
-    _: bool, //gui
+    _: Renderer, //renderer
     version: []const u8,
     all_targets: bool,
     test_filters: []const []const u8,
@@ -171,7 +173,7 @@ fn build_release(
             true, // strip release builds
             use_llvm,
             pie,
-            false, //gui
+            .terminal,
             version,
             test_filters,
         );
@@ -190,7 +192,7 @@ fn build_release(
             false, // don't strip debug builds
             use_llvm,
             pie,
-            false, //gui
+            .terminal,
             version,
             test_filters,
         );
@@ -210,7 +212,7 @@ fn build_release(
                 true, // strip release builds
                 use_llvm,
                 pie,
-                true, //gui
+                .d3d11,
                 version,
                 test_filters,
             );
@@ -229,7 +231,7 @@ fn build_release(
                 false, // don't strip debug builds
                 use_llvm,
                 pie,
-                true, //gui
+                .d3d11,
                 version,
                 test_filters,
             );
@@ -251,7 +253,7 @@ pub fn build_exe(
     strip: bool,
     use_llvm: ?bool,
     pie: ?bool,
-    gui: bool,
+    renderer: Renderer,
     version: []const u8,
     test_filters: []const []const u8,
 ) void {
@@ -259,7 +261,7 @@ pub fn build_exe(
     options.addOption(bool, "enable_tracy", tracy_enabled);
     options.addOption(bool, "use_tree_sitter", use_tree_sitter);
     options.addOption(bool, "strip", strip);
-    options.addOption(bool, "gui", gui);
+    options.addOption(bool, "gui", renderer != .terminal);
 
     const options_mod = options.createModule();
 
@@ -489,8 +491,9 @@ pub fn build_exe(
     });
 
     const renderer_mod = blk: {
-        if (gui) switch (target.result.os.tag) {
-            .windows => {
+        switch (renderer) {
+            .terminal => break :blk tui_renderer_mod,
+            .d3d11 => {
                 const win32_dep = b.lazyDependency("win32", .{}) orelse break :blk tui_renderer_mod;
                 const win32_mod = win32_dep.module("win32");
                 const gui_mod = b.createModule(.{
@@ -526,12 +529,11 @@ pub fn build_exe(
                 });
                 break :blk mod;
             },
-            else => |tag| {
-                std.log.err("OS '{t}' does not support -Dgui mode", .{tag});
+            .gui => {
+                std.log.err("-Drenderer=gui is not yet implemented", .{});
                 std.process.exit(0xff);
             },
-        };
-        break :blk tui_renderer_mod;
+        }
     };
 
     const keybind_mod = b.createModule(.{
@@ -681,7 +683,7 @@ pub fn build_exe(
         },
     });
 
-    const exe_name = if (gui) "flow-gui" else "flow";
+    const exe_name = if (renderer != .terminal) "flow-gui" else "flow";
 
     const exe = b.addExecutable(.{
         .name = exe_name,
@@ -727,7 +729,7 @@ pub fn build_exe(
         exe.addWin32ResourceFile(.{
             .file = b.path("src/win32/flow.rc"),
         });
-        if (gui) {
+        if (renderer != .terminal) {
             exe.subsystem = .Windows;
         }
     }

src/gui/Cell.zig

@@ -0,0 +1,20 @@
+// Shared GPU cell types used by all GPU renderer backends.
+
+pub const Rgba8 = packed struct(u32) {
+    a: u8,
+    b: u8,
+    g: u8,
+    r: u8,
+    pub fn initRgb(r: u8, g: u8, b: u8) Rgba8 {
+        return .{ .r = r, .g = g, .b = b, .a = 255 };
+    }
+    pub fn initRgba(r: u8, g: u8, b: u8, a: u8) Rgba8 {
+        return .{ .r = r, .g = g, .b = b, .a = a };
+    }
+};
+
+pub const Cell = extern struct {
+    glyph_index: u32,
+    background: Rgba8,
+    foreground: Rgba8,
+};

src/gui/GlyphRasterizer.zig

@@ -0,0 +1,50 @@
+// GlyphRasterizer — comptime interface specification.
+//
+// Zig's duck-typing means no vtable is needed. This file documents the
+// interface that every rasterizer implementation must satisfy and can be
+// used as a comptime checker.
+//
+// A rasterizer implementation must provide:
+//
+//   pub const Font = <type>;        // font handle (scale, metrics, etc.)
+//   pub const Fonts = <type>;       // font enumeration (for UI font picker)
+//
+//   pub fn init(allocator: std.mem.Allocator) !Self
+//   pub fn deinit(self: *Self) void
+//
+//   pub fn loadFont(self: *Self, name: []const u8, size_px: u16) !Font
+//       Load a named font at a given pixel height. The returned Font
+//       contains pre-computed metrics (cell_size, scale, ascent_px).
+//
+//   pub fn render(
+//       self: *const Self,
+//       font: Font,
+//       codepoint: u21,
+//       kind: enum { single, left, right },
+//       staging_buf: []u8,
+//   ) void
+//       Rasterize a single glyph into the caller-provided A8 staging buffer.
+//       - staging_buf is zero-filled by the caller before each call.
+//       - For `single`: buffer is cell_size.x * cell_size.y bytes.
+//       - For `left`/`right`: buffer is 2*cell_size.x * cell_size.y bytes.
+//         `left` rasterizes at double width into the full buffer.
+//         `right` places the glyph offset by cell_size.x so that the right
+//         half of the buffer contains the right portion of the glyph.
+
+const std = @import("std");
+
+pub fn check(comptime Rasterizer: type) void {
+    const has_Font = @hasDecl(Rasterizer, "Font");
+    const has_Fonts = @hasDecl(Rasterizer, "Fonts");
+    const has_init = @hasDecl(Rasterizer, "init");
+    const has_deinit = @hasDecl(Rasterizer, "deinit");
+    const has_loadFont = @hasDecl(Rasterizer, "loadFont");
+    const has_render = @hasDecl(Rasterizer, "render");
+
+    if (!has_Font) @compileError("Rasterizer missing: pub const Font");
+    if (!has_Fonts) @compileError("Rasterizer missing: pub const Fonts");
+    if (!has_init) @compileError("Rasterizer missing: pub fn init");
+    if (!has_deinit) @compileError("Rasterizer missing: pub fn deinit");
+    if (!has_loadFont) @compileError("Rasterizer missing: pub fn loadFont");
+    if (!has_render) @compileError("Rasterizer missing: pub fn render");
+}

src/win32/DwriteRenderer.zig

@@ -5,7 +5,7 @@ const win32 = @import("win32").everything;
 const win32ext = @import("win32ext.zig");
 
 const dwrite = @import("dwrite.zig");
-const XY = @import("xy.zig").XY;
+const XY = @import("../gui/xy.zig").XY;
 
 pub const Font = dwrite.Font;
 pub const Fonts = dwrite.Fonts;

src/win32/d3d11.zig

@@ -4,10 +4,11 @@ const win32 = @import("win32").everything;
 const win32ext = @import("win32ext.zig");
 
 const dwrite = @import("dwrite.zig");
-const GlyphIndexCache = @import("GlyphIndexCache.zig");
+const GlyphIndexCache = @import("../gui/GlyphIndexCache.zig");
 const TextRenderer = @import("DwriteRenderer.zig");
 
-const XY = @import("xy.zig").XY;
+const XY = @import("../gui/xy.zig").XY;
+const gui_cell = @import("../gui/Cell.zig");
 
 pub const Font = TextRenderer.Font;
 pub const Fonts = TextRenderer.Fonts;
@@ -31,21 +32,9 @@ const global = struct {
     var background: Rgba8 = .{ .r = 19, .g = 19, .b = 19, .a = 255 };
 };
 
-pub const Color = Rgba8;
-const Rgba8 = packed struct(u32) {
-    a: u8,
-    b: u8,
-    g: u8,
-    r: u8,
-    pub fn initRgb(r: u8, g: u8, b: u8) Color {
-        return .{ .r = r, .g = g, .b = b, .a = 255 };
-    }
-    pub fn initRgba(r: u8, g: u8, b: u8, a: u8) Color {
-        return .{ .r = r, .g = g, .b = b, .a = a };
-    }
-};
-
-pub const Cell = shader.Cell;
+pub const Color = gui_cell.Rgba8;
+const Rgba8 = gui_cell.Rgba8;
+pub const Cell = gui_cell.Cell;
 
 // types shared with the shader
 pub const shader = struct {
@@ -54,11 +43,7 @@ pub const shader = struct {
         col_count: u32,
         row_count: u32,
     };
-    pub const Cell = extern struct {
-        glyph_index: u32,
-        background: Rgba8,
-        foreground: Rgba8,
-    };
+    pub const Cell = gui_cell.Cell;
 };
 
 const swap_chain_flags: u32 = @intFromEnum(win32.DXGI_SWAP_CHAIN_FLAG_FRAME_LATENCY_WAITABLE_OBJECT);

src/win32/gui.zig

@@ -18,10 +18,10 @@ const input = @import("input");
 const windowmsg = @import("windowmsg.zig");
 
 const render = @import("d3d11.zig");
-const xterm = @import("xterm.zig");
+const xterm = @import("../gui/xterm.zig");
 
 const FontFace = @import("FontFace.zig");
-const XY = @import("xy.zig").XY;
+const XY = @import("../gui/xy.zig").XY;
 
 const WM_APP_EXIT = win32.WM_APP + 1;
 const WM_APP_SET_BACKGROUND = win32.WM_APP + 2;