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.

Makefile

@@ -20,10 +20,21 @@ $(SOURCE)/.git/HEAD:
 
 deps-fetch: $(SOURCE)/.git/HEAD
 
+# Zig's `standardOptimizeOption` defaults to .Debug when no
+# -Doptimize is passed, which makes libghostty-vt's CSI/SGR parser
+# an order of magnitude slower — truecolor full-screen frames spend
+# ~16-29 ms each in em.Write under Debug (see
+# internal/app/bench_test.go BenchmarkEmulator_Write_*), which caps
+# the full PTY-to-host pipeline at ~60 fps. ReleaseFast is the
+# right default for the shipped artefact. Override with
+# `make deps GHOSTTY_VT_OPTIMIZE=Debug` when you actually want a
+# debug build of the upstream lib.
+GHOSTTY_VT_OPTIMIZE ?= ReleaseFast
+
 $(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; }
-	@echo ">> building libghostty-vt with zig"
-	@cd $(SOURCE) && zig build -Demit-lib-vt --prefix $(INSTALL)
+	@echo ">> building libghostty-vt with zig (optimize=$(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)"