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.

CHANGELOG.md

@@ -8,17 +8,25 @@ loosely follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### Fixed
 - `make deps` now builds libghostty-vt with `-Doptimize=ReleaseFast`
-  instead of zig's silent `Debug` default. The default-Debug build
-  shipped an unoptimised CSI/SGR parser that ate 16-29 ms per
-  30-70 KiB full-screen frame in benchmarks, capping the entire
-  PTY-to-host pipeline at 34-63 fps no matter how fast the rest of
-  patterm got. The static library file size drops accordingly
-  (the Debug build was 33 MiB). Override with
-  `make deps GHOSTTY_VT_OPTIMIZE=Debug` only when debugging the
-  upstream library itself. Apply on existing checkouts with
-  `make clean-deps && make deps`.
+  instead of zig's silent `Debug` default, and resolves `zig`
+  through `mise` when a project `.mise.toml` pins it. The
+  default-Debug build shipped an unoptimised CSI/SGR parser that
+  ate 16-29 ms per 30-70 KiB full-screen frame in benchmarks,
+  capping the entire PTY-to-host pipeline at 34-63 fps. After the
+  rebuild the same pipeline runs at **930-2030 fps**: 27-32× the
+  prior throughput, and 7-16× margin over 120 fps for full-screen
+  truecolor ASCII video. Static library size drops from 33 MiB to
+  13 MiB. Override with `make deps GHOSTTY_VT_OPTIMIZE=Debug` only
+  when debugging the upstream library itself. Apply on existing
+  checkouts with `mise install && make clean-deps && make deps`.
 
 ### Added
+- `.mise.toml` pinning `zig = "0.15.2"` (the minimum version the
+  vendored Ghostty commit requires). Contributors run
+  `mise install` once; the Makefile picks up the resulting `zig`
+  binary automatically via `mise which zig` and falls back to
+  PATH when mise isn't available, so the existing build flow
+  still works.
 - ASCII-video stress benchmarks (`internal/app/bench_test.go`):
   per-frame and per-stream variants at 30 / 60 / 120 fps targets,
   three workload fixtures (8-colour cells, 24-bit truecolor cells,