casei: close the Unicode field gaps - #13

The original arena omitted multi-pattern enumeration and the hard Unicode shapes that exposed decoded confirmation cost. Rebar found three same-contract losses, including a roughly ninefold gap.

Add tagged multi-pattern candidates, width-aware raw confirmation, an exact common-byte origin gate, and wider AVX-512 schedules. Pair each field timing with its competitor and retain the two-host field, Rebar, ablation, and native-reachability receipts.

The resulting source wins every current arena row and all five same-contract Rebar rows on Ice Lake and Sapphire Rapids while preserving the simple-fold contract and portable fallback.

The library supports Go 1.22, while testing.B.Loop was added in Go 1.24. Keep the diagnostic benchmark and its B.Loop timing semantics without making the ordinary amd64 test file require a newer toolchain.

Go vet checks assembly stack operands against the declaration names. Name the two multi-value results so the existing ret, width, and tags operands match the Go ABI without changing the generated instruction stream.

The scalar confirmation fix changed the source after the previous two-host evidence was captured. Re-run the pinned field and Rebar on qualifying Ice Lake and Sapphire Rapids hosts, and bind the public claims and checksum receipts to the repaired implementation.\n\nBoth hosts win all 36 BenchmarkBar rows across three passes and all five same-contract Rebar rows.

The variable-confirmation test called its internal full-block kernel directly even when runtime dispatch would reject that path. Linux runners without AVX-512 therefore executed a VBMI instruction and trapped.\n\nKeep portable behavior coverage on every host, but make the direct vector assertion conditional on the same feature predicate as production dispatch.

The VBMI-only test guard changed the signed source stream after the previous receipts were recorded. Re-run both qualifying hosts and refresh the field, Rebar, native reachability, checksum, and public summary artifacts.\n\nEvery BenchmarkBar sample remains below 1.0 across 36 rows and three passes per host. The same-contract Rebar selection remains 5 for 5 on both hosts with pinned Hyperscan 5.4.2.