perf: add ghostty-bench and bring terminal throughput to Zig parity

Adds crates/ghostty-bench, a port of ZIG/src/benchmark and src/synthetic:
the ghostty-bench binary (terminal-stream, terminal-parser,
codepoint-width, grapheme-break, is-symbol, osc-parser, screen-clone
with Zig's CLI) and ghostty-synthetic data generators (+ascii, +utf8,
+osc, plus a Rust-only +sgr mode).

Measured against Ghostty's own ghostty-bench built with Zig 0.15.2
(ReleaseFast) vs --profile production, the hot paths were optimized
without behaviour changes:
- Terminal::print single-lookup fast path for width-1 chars mid-line
- page grapheme allocator: bitmap first-fit instead of a linear scan
  (was 38% of utf8 stream time; randomized equivalence test added)
- plain cells skip per-cell work in clear_cells and row cloning
- ASCII fast path + strict hand-written UTF-8 decoder (exhaustive
  equivalence test against std)
- parser/OSC parser split into inlined fast and state-change paths
- RenderState allocates grapheme vectors only for grapheme rows
- Page::assert_integrity no-op stub so the production profile builds

terminal-stream is now at parity (ascii 1.08x, utf8 0.97x, SGR 1.04x of
Zig's time). Still slower: screen-clone (~2.6x, zero-filling fresh
cell Vecs vs Zig's lazily-zeroed mmap), osc-parser (1.9x), and the raw
parser on SGR data (1.5x). Full table in docs/rewrite/notes/perf-P1.md.

Test Plan:
- cargo test --workspace (no failures)
- cargo run --profile production -p ghostty-bench -- +terminal-stream
  --data=<file> and compare with Zig's ghostty-bench

Cargo.lock

@@ -1245,6 +1245,16 @@ dependencies = [
  "wayland-protocols-plasma",
 ]
 
+[[package]]
+name = "ghostty-bench"
+version = "0.1.0"
+dependencies = [
+ "ghostty-terminal",
+ "ghostty-unicode",
+ "ghostty-vt",
+ "log",
+]
+
 [[package]]
 name = "ghostty-cli"
 version = "0.1.0"
@@ -1255,6 +1265,7 @@ dependencies = [
  "ghostty-font",
  "ghostty-input",
  "ghostty-os",
+ "ghostty-vt",
  "log",
  "ratatui",
  "tempfile",
@@ -1269,6 +1280,7 @@ dependencies = [
  "const_format",
  "ghostty-input",
  "ghostty-os",
+ "ghostty-terminal",
  "ghostty-vt",
  "log",
  "onig",
@@ -1339,6 +1351,7 @@ name = "ghostty-os"
 version = "0.1.0"
 dependencies = [
  "gettext-rs",
+ "ghostty-vt",
  "log",
  "nix 0.31.3",
  "rustix",