feat(gui): M2 - sokol_gfx GPU backend initialised from wio window

Implements milestone 2 of the wio+sokol_gfx cross-platform GUI renderer:

- build.zig.zon: add wio and sokol-zig as lazy path dependencies
- build.zig: wire -Drenderer=gui - creates modules for gpu, app, and the
  stub rasterizer; links sokol_clib against system GL/X11/Xi/Xcursor/ALSA
  (requires: libgl-dev libx11-dev libxi-dev libxcursor-dev libasound2-dev)
- src/gui/gpu/builtin.glsl.zig: hand-crafted GLCORE ShaderDesc with a
  full-screen-quad vertex stage and a cell-grid fragment stage; Y-flip via
  row_count*cell_size_y; FsParams as 4 INT uniforms; RGBA32UI cell texture
  and R8 glyph-atlas texture with NONFILTERING samplers
- src/gui/gpu/gpu.zig: sokol_gfx backend mirroring the d3d11 backend;
  TRIANGLE_STRIP pipeline, CPU-side glyph atlas shadow, per-frame cell
  texture update via sg.updateImage
- src/gui/rasterizer/stub.zig: blank-glyph stub rasterizer for M2 testing
- src/gui/wio/app.zig: wio event loop + thespian bridge; creates a core
  OpenGL 3.3 context, calls sg.setup/gpu.init, sends RDR messages for
  WindowCreated/Resize/keyboard/mouse events, renders on screen_pending
- src/gui/wio/input.zig: wio Button → Flow codepoint/mouse-button mapping
- src/renderer/gui/renderer.zig: standard renderer interface consumed by
  tui.zig; dispatches RDR events, delegates font/cursor/clipboard to app

build.zig.zon

@@ -42,6 +42,14 @@
             .hash = "zigwin32-25.0.28-preview-AAAAAICM5AMResOGQnQ85mfe60TTOQeMtt7GRATUOKoP",
             .lazy = true,
         },
+        .wio = .{
+            .path = "../flow-deps/wio",
+            .lazy = true,
+        },
+        .sokol = .{
+            .path = "../flow-deps/sokol-zig",
+            .lazy = true,
+        },
         .diffz = .{
             .url = "git+https://github.com/ziglibs/diffz.git#fbdf690b87db6b1142bbce6d4906f90b09ce60bb",
             .hash = "diffz-0.0.1-G2tlIezMAQBwGNGDs7Hn_N25dWSjEzgR_FAx9GFAvCuZ",