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.