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-bench/src/main.rs

@@ -0,0 +1,49 @@
+//! `ghostty-bench +<action> [--option=value ...]` (port of `main_bench.zig`).
+use ghostty_bench::{args::Args, benches};
+use std::process::ExitCode;
+
+fn main() -> ExitCode {
+    let argv: Vec<String> = std::env::args().skip(1).collect();
+    let mut action = None;
+    for a in &argv {
+        if let Some(name) = a.strip_prefix('+') {
+            if action.is_some() {
+                eprintln!("error: multiple actions specified");
+                return ExitCode::FAILURE;
+            }
+            action = Some(name.to_owned());
+        }
+    }
+    let Some(action) = action else {
+        eprintln!(
+            "error: no action specified; available: {}",
+            benches::ACTIONS.join(", ")
+        );
+        return ExitCode::FAILURE;
+    };
+    let mut args = match Args::parse(argv) {
+        Ok(a) => a,
+        Err(e) => {
+            eprintln!("error: {e}");
+            return ExitCode::FAILURE;
+        }
+    };
+    // Rust-only convenience: print the measured step time to stderr.
+    let time = args.take_flag("time").unwrap_or(false);
+    match benches::main_action(&action, args) {
+        Ok(r) => {
+            if time {
+                eprintln!(
+                    "iterations={} duration_ns={}",
+                    r.iterations,
+                    r.duration.as_nanos()
+                );
+            }
+            ExitCode::SUCCESS
+        }
+        Err(e) => {
+            eprintln!("error: {e}");
+            ExitCode::FAILURE
+        }
+    }
+}