WebAssembly profile: SIMD

Parent: #142 Depends on: #262 Roadmap: #134

Implement the pinned WebAssembly SIMD profile end to end.

Acceptance:

• Pin the selected SIMD specification/test revision and opcode inventory.
• Implement v128 decoding, validation, locals/stack representation, memory operations, and every selected SIMD instruction.
• Preserve lane bits exactly, including NaN payloads and signed zero.
• Declare supported architectures and use correct portable fallback behavior.
• Publish upstream SIMD corpus totals and reproducible performance evidence.

Child issue map

• #279 — v128 type, binary, validator, and runtime foundation
• #280 — memory, lane, shuffle, and bitwise operations
• #281 — integer lane arithmetic and comparisons
• #282 — floating lanes and numeric conversions
• #283 — exact upstream score and performance evidence

Implementation order

Land #279 first. Memory/lane operations #280 establish the shared lane helpers; integer #281 and float #282 may then proceed independently. Terminal evidence #283 closes only after every instruction family is complete and compared against the portable scalar oracle.

Foundation child #279 is complete through a3b8ea3. Exact surface: 236 legal opcodes, 20 reserved holes, 56 pinned corpus files, six immediate shapes, complete static validation, exact 128-bit runtime slots, and explicit JS/test-only boundaries. Execution proceeds next in #280; #281/#282 remain dependent on its shared lane helpers.

The complete fixed-width SIMD profile is delivered.

All child issues #279–#283 are closed. The terminal pinned proposal score is 25,466 / 25,466 applicable commands across all 56 files, with 510 explicit text-format n/a commands and zero failures or runner errors. All 236 selected opcodes decode, validate, and execute with exact lane/NaN handling through the portable architecture-independent path.

The committed terminal inventory, bounded CI, complete reproduction, 224-sample scalar/JSC benchmark, raw artifacts, architecture/fallback declaration, README, and detailed docs are linked from #283 and landed through be438ae.