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/render.rs

@@ -549,7 +549,6 @@ impl RenderState {
                 continue;
             }
 
-            row.cells.grapheme.resize_with(cols, Vec::new);
             row.cells.style.resize(cols, Style::default());
             for (x, page_cell) in page_cells.iter().enumerate() {
                 // Append assuming its a single-codepoint, styled cell
@@ -569,6 +568,11 @@ impl RenderState {
                         if let Ok(idx) = p.cell_index(x, usize::from(row_pin.y))
                             && let Some(g) = p.lookup_grapheme(idx)
                         {
+                            // Only rows that actually contain graphemes pay
+                            // for the per-cell vectors.
+                            if row.cells.grapheme.is_empty() {
+                                row.cells.grapheme.resize_with(cols, Vec::new);
+                            }
                             row.cells.grapheme[x].extend_from_slice(g);
                         }
                     }