[mitchellh]

This PR speeds up our formatting (plain text, html, and VT) by anywhere from ~1.5x to ~8x.

The formatter is the hot path behind multiple features in Ghostty GUI: clipboard copy (plain/VT/HTML), write_screen_file, selectionString, and terminal search sliding window. It's also the hot path for libghostty users, namely people like zmx which utilize the VT formatter to restore a terminal.

This PR also adds the benchmarking infrastructure for the formatter.

How

• Fast cell-run optimization. For simple cells (single codepoint, no style/hyperlink) we encode them as a single run rather than one at a time.
• Make some arguments comptime. Generates more code but benchmarks show it improves things, specifically for per-format switches that we do a LOT.
• Interned style id fast path. Styles are interned per page, so id equality implies style equality. We track the id of the active style and skip the per-cell Style copy + eql when it matches.
• Fast printing. Avoid std.fmt where possible and assemble integers, RGB colors, codepoints in fixed-width buffers with a single memcpy. This was extracted partially to fastprint.zig so we can reuse it.
• Avoid double-formatting for tracked pins. Previously we formatted twice (once through a Discarding writer to count bytes) for pin maps. Now I'm smarter about it and do a single pass.

Performance

All on my machine, 80x24 terminal, 10K lines of scrollback.

workload	main	this PR	speedup	throughput
plain / plain	5.74 ms	1.67 ms	3.4x	364 MB/s
plain / vt	6.46 ms	1.04 ms	6.2x	596 MB/s
plain / html	7.39 ms	2.32 ms	3.2x	308 MB/s
unicode / plain	9.74 ms	5.42 ms	1.8x	276 MB/s
unicode / vt	10.42 ms	5.53 ms	1.9x	275 MB/s
unicode / html	12.53 ms	7.35 ms	1.7x	509 MB/s
styled / plain	5.65 ms	1.69 ms	3.4x	360 MB/s
styled / vt	9.07 ms	4.20 ms	2.2x	409 MB/s
styled / html	10.78 ms	6.64 ms	1.6x	740 MB/s
mixed / plain	8.59 ms	4.81 ms	1.8x	226 MB/s
mixed / vt	11.25 ms	6.65 ms	1.7x	250 MB/s
mixed / html	14.47 ms	10.52 ms	1.4x	414 MB/s
wrapped / plain	7.30 ms	1.11 ms	6.6x	733 MB/s
wrapped / vt	8.14 ms	1.04 ms	7.8x	789 MB/s
wrapped / html	9.00 ms	2.12 ms	4.2x	465 MB/s
pin-map / plain	12.51 ms	3.80 ms	3.3x
pin-map / vt	13.12 ms	2.99 ms	4.4x
active screen / plain	12.5 µs	2.7 µs	4.6x
active screen / vt	18.4 µs	6.8 µs	2.7x

Workloads:

• plain is ASCII lines
• unicode is 2/3/4-byte codepoints with 10% grapheme clusters
• styled is heavy SGR churn
• mixed is styles + Unicode + hyperlinks
• wrapped is a continuous soft-wrapped stream
• pin-map/active screen are the selectionString/search-style and visible-screen-only cases respectively.