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.

.mise.toml

@@ -0,0 +1,8 @@
+# mise config — `mise install` provisions the tools `make deps` needs.
+#
+# libghostty-vt is built from a pinned upstream Ghostty commit; that
+# commit's build.zig.zon pins minimum_zig_version = 0.15.2. We match
+# it here so contributors don't have to puzzle out the version from
+# a deep upstream file.
+[tools]
+zig = "0.15.2"

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
+  instead of zig's silent `Debug` default, and resolves `zig`
-  shipped an unoptimised CSI/SGR parser that ate 16-29 ms per
+  through `mise` when a project `.mise.toml` pins it. The
-  30-70 KiB full-screen frame in benchmarks, capping the entire
+  default-Debug build shipped an unoptimised CSI/SGR parser that
-  PTY-to-host pipeline at 34-63 fps no matter how fast the rest of
+  ate 16-29 ms per 30-70 KiB full-screen frame in benchmarks,
-  patterm got. The static library file size drops accordingly
+  capping the entire PTY-to-host pipeline at 34-63 fps. After the
-  (the Debug build was 33 MiB). Override with
+  rebuild the same pipeline runs at **930-2030 fps**: 27-32× the
-  `make deps GHOSTTY_VT_OPTIMIZE=Debug` only when debugging the
+  prior throughput, and 7-16× margin over 120 fps for full-screen
-  upstream library itself. Apply on existing checkouts with
+  truecolor ASCII video. Static library size drops from 33 MiB to
-  `make clean-deps && make deps`.
+  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,

Makefile

@@ -31,10 +31,19 @@ deps-fetch: $(SOURCE)/.git/HEAD
 # debug build of the upstream lib.
 GHOSTTY_VT_OPTIMIZE ?= ReleaseFast
 
+# Resolve zig via the project's mise pin (.mise.toml) when available,
+# falling back to whatever's on PATH. mise keeps the zig version in
+# lockstep with what the pinned ghostty commit requires; without it,
+# contributors have to chase the version requirement themselves.
+ZIG := $(shell command -v mise >/dev/null && mise which zig 2>/dev/null || command -v zig 2>/dev/null)
+
 $(INSTALL)/lib/libghostty-vt.a: $(SOURCE)/.git/HEAD
-	@command -v zig >/dev/null || { echo "ERROR: zig not on PATH (need >=0.15.2 to build libghostty-vt)"; exit 1; }
+	@if [ -z "$(ZIG)" ]; then \
-	@echo ">> building libghostty-vt with zig (optimize=$(GHOSTTY_VT_OPTIMIZE))"
+		echo "ERROR: zig not available. Run \`mise install\` (see .mise.toml — needs zig 0.15.2) or install zig manually."; \
-	@cd $(SOURCE) && zig build -Demit-lib-vt -Doptimize=$(GHOSTTY_VT_OPTIMIZE) --prefix $(INSTALL)
+		exit 1; \
+	fi
+	@echo ">> building libghostty-vt with $(ZIG) (optimize=$(GHOSTTY_VT_OPTIMIZE))"
+	@cd $(SOURCE) && $(ZIG) build -Demit-lib-vt -Doptimize=$(GHOSTTY_VT_OPTIMIZE) --prefix $(INSTALL)
 	@test -f $(INSTALL)/lib/libghostty-vt.a || { echo "ERROR: expected static lib at $(INSTALL)/lib/libghostty-vt.a"; exit 1; }
 	@echo ">> libghostty-vt installed under $(INSTALL)"
 

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
+Ryzen 7 7800X3D, libghostty-vt **ReleaseFast** after the Makefile
-below):
+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_8Color_120fps     493 µs/frame  2030 fps_ceiling   5.9% budget
-Pipeline_ASCIIVideo_TrueColor_120fps 29224 µs/frame    34 fps_ceiling  350% 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_8Color_120fps    257 µs/frame  3890 fps_ceiling
-Emulator_Write_Stream_TrueColor_120fps  29241 µs/frame    34 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
+Result of the fix below: 27-32× pipeline speedup, 60× emulator
-caps at 34-63 fps. **The whole gap is libghostty-vt's em.Write — its
+speedup. Pipeline hits 930-2030 fps end-to-end — 7-16× headroom
-parser is shipping in Debug mode, which is also a 33 MiB static
+over the 120 fps target on the heaviest workload (truecolor
-library file (release builds are a fraction of that).**
+full-screen redraws).
-
-- [ ] **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.
 
 
 - [ ] **viewport renderer allocates ~1 alloc per 4 input bytes on SGR/CSI-heavy chunks.** [MEDIUM]