WebAssembly Core 3: implement the relaxed SIMD opcode set

[chrisbbreuer]

Parent: #366
Related: #264, #142
Roadmap: #134

The stable wg-3.0 tree includes seven test/core/relaxed-simd/*.wast files. zig-js now implements the complete relaxed-SIMD binary, validation, and portable execution surface against that exact corpus.

Acceptance:

• Pin and inventory every relaxed-SIMD opcode and immediate from wg-3.0.
• Decode and validate the full opcode set behind an explicit relaxed_simd feature gate.
• Execute every allowed nondeterministic result without over-constraining platform behavior.
• Preserve architecture-independent interpreter fallback and bit-exact lanes where the spec requires them.
• Pass every applicable command in all seven exact wg-3.0 relaxed-SIMD files.
• Add focused units, bounded CI witnesses, feature registry entries, and documentation.

Evidence:

• Exact source: WebAssembly/spec@9d36019973201a19f9c9ebb0f10828b2fe2374aa (wg-3.0).
• Inventory: 20 contiguous 0xfd subopcodes, 0x100 through 0x113.
• Corpus: 77/77 pass, 0 fail, 0 N/A, 0 runner errors across all seven files.
• Harness: 18/18 Python regression tests, including nested vector either alternatives.
• Decoder slice: 38/38 pass after moving the former oversized witness from newly-valid 0x100 to 0x114.
• Consolidated project run: 1,219/1,220 before that stale test expectation; the only failure was the obsolete 0x100 expectation, and its complete 38-test owning slice passed after correction.
• Registry/matrix verifier: green, 14 pinned features and the unchanged 87,838-command published matrix.

Landed on main:

• b1862778 feat(wasm): implement relaxed SIMD (WebAssembly Core 3: implement the relaxed SIMD opcode set #386)
• 7229d3eb ci(wasm): gate Core 3 relaxed SIMD (WebAssembly Core 3: implement the relaxed SIMD opcode set #386)
• e99f0654 docs(wasm): publish relaxed SIMD evidence (WebAssembly Core 3: implement the relaxed SIMD opcode set #386)

[chrisbbreuer]

Implemented and pushed directly to main. The exact pinned WG-3.0 relaxed-SIMD corpus is green at 77/77 with no failures, exclusions, or runner errors.