bench: terminal-stream reads 64KiB chunks to match the IO thread

The terminal-stream benchmark fed the stream in 4KiB chunks while
the real IO thread reads from the pty into 64KiB buffers (see
buffer_capacity in termio/Exec.zig) and hands those to the stream
whole. Chunk size affects measurement in two ways: it determines
how often the stream crosses a chunk boundary (partial UTF-8
sequences, escape sequences split mid-parse) and how many read
syscalls the harness itself performs (a 2.6 GB corpus is ~636k
pread calls at 4KiB versus ~40k at 64KiB).

This bumps the benchmark read and dispatch buffers to 64KiB so the
stream is exercised with realistic chunk sizes. Measured with
ghostty-bench terminal-stream on a 2.6 GB recording of real
terminal sessions (120x80, M4 Max, ReleaseFast, hyperfine means):

| harness      | time            | throughput |
|--------------|-----------------|------------|
| 4KiB chunks  | 9.651 s ± 0.013 | 270 MB/s   |
| 64KiB chunks | 9.582 s ± 0.101 | 272 MB/s   |

The stream itself is barely chunk-size sensitive (most time is in
parsing and terminal state updates), but the harness now matches
what the IO thread actually does, and later commits are measured
against this configuration.

src/benchmark/TerminalStream.zig

@@ -112,11 +112,14 @@ fn step(ptr: *anyopaque) Benchmark.Error!void {
     // aren't currently IO bound.
     const f = self.data_f orelse return;
 
-    var read_buf: [4096]u8 align(std.atomic.cache_line) = undefined;
+    var read_buf: [64 * 1024]u8 align(std.atomic.cache_line) = undefined;
     var f_reader = f.reader(&read_buf);
     const r = &f_reader.interface;
 
-    var buf: [4096]u8 = undefined;
+    // This buffer size matches the read buffer size used by the
+    // real IO thread (see termio Exec.zig buffer_capacity) so that
+    // the benchmark exercises the stream with realistic chunk sizes.
+    var buf: [64 * 1024]u8 = undefined;
     while (true) {
         const n = r.readSliceShort(&buf) catch {
             log.warn("error reading data file err={?}", .{f_reader.err});