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-terminal/src/page_list.rs

@@ -547,6 +547,19 @@ impl PageList {
         }
         Ok(())
     }
+    /// Mutable access to a tracked pin's location for hot paths that only
+    /// change `x` within the same row (cursor movement while printing).
+    /// Unlike [`PageList::set_tracked_pin`] this performs no node validation
+    /// and does not invalidate the cached viewport offset, so it must not be
+    /// used to move the pin to another row or node.
+    #[inline]
+    pub fn tracked_pin_x_mut(&mut self, h: TrackedPin) -> Option<&mut Pin> {
+        self.pins
+            .get_mut(h.index)
+            .filter(|s| s.generation == h.generation)?
+            .pin
+            .as_mut()
+    }
     /// Release a tracked handle. The internal viewport handle remains owned.
     pub fn untrack_pin(&mut self, h: TrackedPin) {
         if self.viewport_pin == Some(h) {