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"