WebAssembly threads: atomic opcode inventory, decode, and validation

[chrisbbreuer]

Parent: #265
Depends on: #262
Roadmap: #142, #134

Define the exact pinned WebAssembly threads instruction surface and implement its binary and validator foundation.

Acceptance:

• Check in the exact 0xfe atomic opcode/immediate inventory from the pinned proposal revision.
• Decode every selected atomic instruction and fence immediate with deterministic malformed-binary diagnostics.
• Validate stack signatures, memory presence, exact natural alignment, and feature gates; keep fence valid without memory.
• Accept atomic access and notify on ordinary memories, leave the specified wait-on-unshared trap to runtime issue WebAssembly threads: execute atomics, wait, notify, and fence #286, and reject shared limits without a maximum.
• Add focused decoder/validator tests and registry drift checks.

[chrisbbreuer]

Completed the pinned binary and validator foundation.

• 98911c4 checks in the exact 67-opcode 0xfe inventory from WebAssembly/threads@979d0fc, including the 13-file corpus count, reserved 0x04–0x0f gap, immediates, alignments, and stack shapes. The registry verifier proves exact source/enum agreement.
• 040d0c0 decodes shared limits and every atomic/fence immediate, requires maximum on shared memory, enforces exact natural alignment and all operand/result signatures, retains fence-without-memory validity, and distinguishes the runtime-only wait-on-unshared trap.
• 867aed0 publishes the current boundary and child sequence in README/docs. Execution deliberately traps rather than exposing partial semantics until WebAssembly threads: shared Memory backing and JavaScript API ownership #285/WebAssembly threads: execute atomics, wait, notify, and fence #286 land.

Verification:

• atomic.zig standalone test: 1/1 green
• feature registry checker: green
• focused decode suite: 19/19 green
• focused validator suite: 47/47 green
• pinned upstream appendix/inventory subopcode comparison: 67/67 exact

The next dependency is #285 stable shared backing and JavaScript API ownership.