perf: optimize header parsing and encoding

[geeknoid]

Reduce parser, iterator, and formatter work without changing the public API.

Optimizations:

• Parse Content-Length and shared second values directly from bytes, with an overflow-free short-number path and checked fallback.
• Remove the redundant HeaderValue clone from Host decoding.
• Specialize quote-free Cache-Control splitting, fold directives directly, and emit directives without a temporary option array.
• Parse Content-Range with byte delimiters and direct integer conversion.
• Parse Range bounds from bytes, streamline satisfiable-range iteration, and consume constructed Strings without copying.
• Use the standard byte-search implementation for ETag validation and scan valid ETag lists once.
• Replace the STS iterator pipeline with a direct byte parser.
• Use byte suffix and prefix scans for Transfer-Encoding and Content-Disposition queries.
• Scan Referrer-Policy values backward to find the last recognized policy.
• Parse Cookie pairs in one pass with documented checked invariants around the retained unchecked slices.
• Preallocate formatting Strings and move them into HeaderValue without copying.
• Add matching 49-case Gungraun and nightly wall-clock benchmark suites.

Benchmark results:

Benchmark	Instructions before	Instructions after	Wall ns before	Wall ns after
Content-Length decode	354	236	14.59	6.89
Age decode	254	193	7.62	3.92
Host decode	790	486	50.68	44.56
Cache-Control decode	2121	1451	163.96	130.64
Cache-Control quoted decode	2628	2366	266.87	174.87
Cache-Control 16-directive decode	6314	5364	528.41	686.60
Content-Range decode	1405	528	91.59	38.48
STS decode	1684	1182	107.53	72.70
STS quoted decode	1741	1356	111.65	92.73
Referrer-Policy decode	2768	1556	200.70	151.34
ETag 32-byte decode	407	369	35.95	37.97
ETag 64-byte decode	567	403	51.47	40.37
ETag 128-byte decode	887	471	79.86	40.85
ETag 256-byte decode	1527	607	115.84	51.63
Transfer-Encoding single query	425	282	25.45	13.67
Transfer-Encoding list query	683	515	57.61	29.59
If-Match single query	1425	807	97.39	49.73
If-Match list query	2333	1392	162.46	88.20
Range satisfiable iteration	2692	1547	222.45	117.73
Cookie lookup	3418	2080	245.11	146.14
Cookie length	3248	2038	222.18	126.66
Content-Disposition query	1245	1104	78.34	61.66
Range construction	1099	1032	97.22	91.62
Cache-Control encode	2114	1610	214.40	125.50
Cache-Control all-directive encode	6576	5685	595.82	588.73
STS encode	1732	1416	184.39	116.87
Content-Range encode	2256	1928	216.36	151.52

Instruction counts are isolated Callgrind measurements. Wall-clock values are representative nightly libtest estimates and are inherently noisier.