[andreybolonin]

Response header splitting scans for : one byte at a time. Use bytes.IndexByte to find the same first separator while keeping the existing key/value copies and space-only trimming.

Add table tests, a differential fuzz test, and a direct splitting benchmark, including missing separators, embedded NUL bytes, tabs, and additional colons.

Initial standalone measurements (Go 1.27.3, Apple M2, darwin/arm64, one 150ms sample per case):

Header	Before	After	Time reduction	Go B/op	allocs/op
X-Request-Id	24.44 ns	20.31 ns	16.9%	32 → 32	2 → 2
Access-Control-Allow-Credentials	35.68 ns	22.99 ns	35.6%	36 → 36	2 → 2
Content-Type	25.42 ns	24.33 ns	4.3%	32 → 32	2 → 2

These exploratory helper measurements are not a statistical comparison or a whole-request speedup claim. Benchmarks were not rerun for this PR.

Validation: gofmt and git diff --check passed. The exact CGO helper and its added tests passed with -race in an isolated harness; differential fuzzing passed 631,569 executions, and go vet passed. Full FrankenPHP tests were not run locally because the required PHP ZTS/shared-library and watcher setup is unavailable.