WebAssembly profiles: version feature registry and deterministic gates

[chrisbbreuer]

Parent: #142
Roadmap: #134

Build the shared foundation for every post-MVP WebAssembly profile.

Acceptance:

• Check in a machine-readable registry naming each supported proposal revision, dependencies, default state, and host/architecture constraints.
• Add explicit decoder/validator feature gates; disabled proposal opcodes, sections, types, and flags fail with stable diagnostics.
• Expose embedder/test configuration without silently enabling features from module contents.
• Verify dependency closure and reject invalid profile combinations before module decoding.
• Add drift tests that fail when implementation gates and the registry disagree.
• Document the versioning and compatibility policy in README/docs.

[chrisbbreuer]

Implemented and verified on main.

Evidence:

• Registry and verifier: 5f544155, 9476a8c0, 57d8a8a4, 86a80e40
• CI drift gate: 400484e3
• Runtime/context decoder and validator gates: 3f7491e2
• Published docs and evidence: af10c01c
• zig build wasm-feature-profiles-check: 4 profiles / 12 features verified
• Focused WebAssembly suite: 126/126 passed, 0 skipped, 0 failed, 0 leaked

The registry remains intentionally default-MVP; post-MVP implementations continue under #263-#268.