mitchell's touchups

- benchmark: avoid buffers to avoid a memcpy
- build: keep frame pointers on macOS. There was some debug changes from
  Zig 0.15 and this helps. Also, Apple actually requires/expects x29 to
  always be a frame pointer.
- build/macos: force libSystem symbols instead of compiler-rt
- global: add InitOpts.tool so that ghostty-gen/bench can parse their
  own actions in `+action`
- quirks: provide our own vectorized memset. see the comment for more
  details why.
- synthetic: fix UB by accessing global.io before it was initialized
- terminal/hash_map: force inline for unique repr types. Zig 0.15
  inlined and 0.16 doesn't, measured a huge slowdown in hyperlink
  benchmarks.
- terminal: add explicit `@Vector` usage for storing a run of identical cells
  as well as for scanning printable cells. This auto-vectorized in Zig
  0.15 but not in Zig 0.16. This produces the same assembly.
- unicode: properties and LUT need power-of-two backing integer to avoid
  bad LLVM codegen

src/main_bench.zig

@@ -1,3 +1,9 @@
 const benchmark = @import("benchmark/main.zig");
 
+comptime {
+    // Force-reference our memset override so its export is emitted.
+    // See quirks_memset.zig for details on why this exists.
+    _ = @import("quirks_memset.zig");
+}
+
 pub const main = benchmark.cli.main;