Exercise the arm64 NEON header parser in CI

[FumingPower3925]

protocol/h1/findheader_arm64.s (NEON) is compiled on arm64 but never functionally exercised — the header-scan unit/fuzz suite only runs the amd64 path. Add an arm64 job (or run the parser suite on macos-latest) so the SIMD path is covered.

[FumingPower3925]

Resolved in v1.6.0 by removal: #697, plus goceleris/docs#73 for the site. There is no NEON header parser left to exercise, because nothing ever called it after 96581bc.

Evidence at 9f4d89b

The script is evidence/celeris-673-679-653-424/lane-20260926/424/callers-at-base.sh.

• findHeaderEnd (protocol/h1/findheader_*) is the only assembly in the tree. git grep -w findHeaderEnd over non-test files finds only:
  • the three declarations (amd64, arm64, generic)
  • the two asm TEXT symbols and their header comments
  • one comment in parser.go
• A call-site query finds no call. The query is the name followed by (, in non-test Go files, excluding the func lines. No go:linkname names the function.
• Positive controls for that query:
  • it finds the live p.parseHeaders(req) at parser.go:118
  • at 96581bc~1 it finds the call that commit removed: parser.go:93: if findHeaderEnd(remaining) < 0
• 96581bc (h1: eliminate redundant upfront findHeaderEnd whole-block scan (headers CRLF-scanned twice/req) #359, 2026-06-17) removed the last caller. This issue was filed on 2026-07-20.
• A note on the method: my first version of the query used git grep -w. -w also needs a non-word character after the match, and the match ends in ( followed by an argument name, so it silently dropped every call. Both positive controls came back empty. The query now spells out its left boundary.

What #697 does

• Deletes the six findheader files, the five TestFindHeaderEnd_* tests and the two benchmarks.
• Changes README.md:35 from "SIMD HTTP parser — SSE2 (amd64) and NEON (arm64) with a generic SWAR fallback" to a zero-copy parser bullet taken from the package doc. There was never a SWAR path.
• docs#73 replaces the "SIMD parser" card at src/pages/index.astro:75.

Verification

• After the deletion, go build ./... and go vet ./protocol/... pass for linux/{amd64,arm64,386,riscv64} and darwin/{amd64,arm64}. These cover every architecture that had an implementation.

• go test -v ./protocol/h1 gives 140 PASS, 0 FAIL, 0 SKIP. go mod tidy -diff is clean.

• Control: with a call put back into parser.go, the build fails on amd64 and arm64 with undefined: findHeaderEnd.

• CI on refactor(h1): delete the unused SIMD header scan and stop advertising a SIMD parser (celeris#424) #697: run 36241726098, 9/9 jobs green. docs#73: build green (run 36241857974).

This comment only reports. The issue closes when #697 merges.