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

crates/ghostty-vt/src/osc/mod.rs

@@ -458,6 +458,7 @@ impl Parser {
         self.writer != Writer::None
     }
 
+    #[inline]
     fn write_byte(&mut self, c: u8) {
         match self.writer {
             Writer::None => unreachable!(),
@@ -474,13 +475,14 @@ impl Parser {
     }
 
     /// Consume the next character c and advance the parser state.
+    ///
+    /// The payload-accumulation fast path is inlined into callers; the
+    /// numeric-prefix state machine lives in [`Parser::next_state`].
+    #[inline]
     pub fn next(&mut self, c: u8) {
-        #[allow(clippy::enum_glob_use)]
-        use State::*;
-
         // If the state becomes invalid for any reason, just discard
         // any further input.
-        if self.state == Invalid {
+        if self.state == State::Invalid {
             return;
         }
 
@@ -491,6 +493,14 @@ impl Parser {
             return;
         }
 
+        self.next_state(c);
+    }
+
+    #[inline(never)]
+    fn next_state(&mut self, c: u8) {
+        #[allow(clippy::enum_glob_use)]
+        use State::*;
+
         match self.state {
             // handled above, so should never be here
             Invalid => unreachable!(),