feat(gui): M3 - TrueType rasterizer with fontconfig font discovery

Replace the M2 stub rasterizer with a real font rasterizer:

- build.zig.zon: add TrueType as a lazy path dependency
- build.zig: swap stub_rasterizer_mod for truetype_rasterizer_mod; link
  fontconfig + libc on Linux; TrueType dep is lazy (falls back gracefully)
- src/gui/rasterizer/truetype.zig: andrewrk/TrueType rasterizer; loadFont
  uses fontconfig to locate the font file, derives cell dimensions from
  vertical metrics (ascent−descent) and 'M' advance width; render blits the
  A8 glyph bitmap into the caller-provided staging buffer with correct
  baseline placement (ascent_px + off_y) and double-wide support (+cell_w
  x-offset for kind=.right); arena allocator per render call
- src/gui/rasterizer/font_finder.zig: OS dispatcher (Linux only for now)
- src/gui/rasterizer/font_finder/linux.zig: fontconfig C API - FcFontMatch
  resolves a font name pattern to an absolute file path

Requires: libfontconfig-dev  (already present alongside libgl-dev etc.)

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

build.zig.zon

@@ -50,6 +50,10 @@
             .path = "../flow-deps/sokol-zig",
             .lazy = true,
         },
+        .TrueType = .{
+            .path = "../flow-deps/TrueType",
+            .lazy = true,
+        },
         .diffz = .{
             .url = "git+https://github.com/ziglibs/diffz.git#fbdf690b87db6b1142bbce6d4906f90b09ce60bb",
             .hash = "diffz-0.0.1-G2tlIezMAQBwGNGDs7Hn_N25dWSjEzgR_FAx9GFAvCuZ",