mise-pin zig 0.15.2; rebuild libghostty-vt ReleaseFast — 27-32x pipeline speedup

Added .mise.toml pinning zig = "0.15.2" (the minimum the vendored
Ghostty commit requires) and taught the Makefile to resolve zig
through mise when available, falling back to PATH. Contributors run
`mise install` once and `make deps` just works.

Re-ran the pipeline benchmarks after rebuilding libghostty-vt with
ReleaseFast (same hardware, AMD Ryzen 7 7800X3D):

                                Debug         ReleaseFast    speedup
  Pipeline 8-colour @120fps     63 fps         2030 fps       32x
  Pipeline truecolor @120fps    34 fps          931 fps       27x
  Emulator-only truecolor       34 fps         2051 fps       60x

7-16x headroom over 120 fps for the heaviest workload (truecolor
full-screen redraws). Static library size 33 MiB -> 13 MiB.

TODO.md baseline numbers updated to reflect post-fix throughput;
the "Debug-mode lib" finding is folded into the result it produced
rather than left as an open item.

TODO.md

@@ -3,8 +3,8 @@ Findings from a codebase sweep — not user-reported, needs review before
 action. Each item names the anchor and a sketched fix.
 
 Baseline benchmark numbers (`go test -bench=. ./internal/app/`, AMD
-Ryzen 7 7800X3D, libghostty-vt **Debug-mode** — see the first item
-below):
+Ryzen 7 7800X3D, libghostty-vt **ReleaseFast** after the Makefile
+fix landed):
 
 ```
 # Renderer alone
@@ -19,35 +19,18 @@ ASCIIVideo_Stream_8Color_120fps     260 µs/frame  3845 fps_ceiling   3.1% budge
 ASCIIVideo_Stream_TrueColor_120fps  576 µs/frame  1735 fps_ceiling   6.9% budget
 
 # Full pipeline (em.Write + renderer + io.Discard write)
-Pipeline_ASCIIVideo_8Color_120fps    15838 µs/frame    63 fps_ceiling  190% budget
-Pipeline_ASCIIVideo_TrueColor_120fps 29224 µs/frame    34 fps_ceiling  350% budget
+Pipeline_ASCIIVideo_8Color_120fps     493 µs/frame  2030 fps_ceiling   5.9% budget
+Pipeline_ASCIIVideo_TrueColor_120fps 1075 µs/frame   931 fps_ceiling  12.9% budget
 
 # Emulator alone (libghostty-vt CSI/SGR parser)
-Emulator_Write_Stream_8Color_120fps     15930 µs/frame    63 fps_ceiling
-Emulator_Write_Stream_TrueColor_120fps  29241 µs/frame    34 fps_ceiling
+Emulator_Write_Stream_8Color_120fps    257 µs/frame  3890 fps_ceiling
+Emulator_Write_Stream_TrueColor_120fps 488 µs/frame  2051 fps_ceiling
 ```
 
-The renderer alone hits 1700-3800 fps with margin. The full pipeline
-caps at 34-63 fps. **The whole gap is libghostty-vt's em.Write — its
-parser is shipping in Debug mode, which is also a 33 MiB static
-library file (release builds are a fraction of that).**
-
-- [ ] **libghostty-vt was being built in Debug mode.** [HIGH — partially fixed]
-  - `Makefile` used `zig build -Demit-lib-vt` with no
-    `-Doptimize`. Zig's `standardOptimizeOption` defaults to
-    `.Debug`, so the shipped static lib was unoptimised. Effect:
-    the SGR/CSI parser eats 16-29 ms per 30-70 KiB full-screen
-    frame, capping the entire patterm pipeline at 34-63 fps. The
-    Makefile now defaults to `ReleaseFast` (override via
-    `make deps GHOSTTY_VT_OPTIMIZE=Debug` if you ever need a
-    debug build of the upstream lib for diagnosing a bug in it).
-  - To apply: `make clean-deps && make deps`, then re-run
-    `go test -bench=BenchmarkPipeline -benchmem ./internal/app/`
-    and confirm the truecolor 120fps stream drops well under 100%
-    budget. Update the numbers in this section after rebuilding.
-  - Severity HIGH because it's the single biggest perf win on the
-    table; the renderer optimisations below are second-order until
-    this lands.
+Result of the fix below: 27-32× pipeline speedup, 60× emulator
+speedup. Pipeline hits 930-2030 fps end-to-end — 7-16× headroom
+over the 120 fps target on the heaviest workload (truecolor
+full-screen redraws).
 
 
 - [ ] **viewport renderer allocates ~1 alloc per 4 input bytes on SGR/CSI-heavy chunks.** [MEDIUM]