Stress-test ASCII video at 30/60/120 fps; fix libghostty-vt Debug build

Added a full ASCII-video benchmark suite that hammers the renderer
with 30 KiB / 70 KiB full-screen frames at 30, 60, and 120 fps
targets — both renderer-only and full-pipeline (em.Write + renderer
+ stdout). Each stream benchmark reports µs/frame, fps_ceiling, and
percent of the per-frame budget consumed.

The pipeline benchmarks revealed we were missing 120 fps by a wide
margin (190%-350% of budget at 120fps, 60-90 fps ceiling). Isolating
em.Write confirmed libghostty-vt is the bottleneck — 16-29 ms per
truecolor frame, library file at 33 MiB.

Root cause: the Makefile invoked `zig build` with no
-Doptimize, and Zig's standardOptimizeOption defaults to Debug. So
the shipped libghostty-vt was unoptimised. Fixed by pinning
ReleaseFast in the Makefile (override via GHOSTTY_VT_OPTIMIZE for
debug builds of the upstream lib).

Existing checkouts need `make clean-deps && make deps` to pick up
the rebuild.

TODO.md

@@ -3,16 +3,53 @@ 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):
+Ryzen 7 7800X3D, libghostty-vt **Debug-mode** — see the first item
+below):
 
 ```
+# Renderer alone
 ViewportRenderer_PlainASCII       229 MB/s     1.3 KB/op    6 allocs/op
 ViewportRenderer_StyledLines       89 MB/s    91   KB/op  4325 allocs/op
 ViewportRenderer_RatatuiBurst      40 MB/s   365   KB/op 17306 allocs/op
 RendererThroughput_ReuseInstance   90 MB/s   316   KB/op 17380 allocs/op
 ContainsOSC_NoOSC                3050 MB/s     0   B/op     0 allocs/op
+
+# ASCII-video stream (renderer only — 3 sec at the target fps)
+ASCIIVideo_Stream_8Color_120fps     260 µs/frame  3845 fps_ceiling   3.1% budget
+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
+
+# 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
 ```
 
+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.
+
+
 - [ ] **viewport renderer allocates ~1 alloc per 4 input bytes on SGR/CSI-heavy chunks.** [MEDIUM]
   - `internal/app/viewport_renderer.go` — the styled-lines and
     ratatui benchmarks show 4-17k allocs per chunk. The hot