Parent: #142 Depends on: #262 Roadmap: #134
Implement the independent Core 2.0 numeric proposals before the mutually dependent structural baseline in #263.
Acceptance:
- Decode all five sign-extension instructions and all eight saturating float-to-integer conversion instructions behind their registry feature gates.
- Validate each instruction with exact operand/result stack types.
- Execute boundary, NaN, infinity, signed, and unsigned cases without traps.
- Add focused binary, validator, interpreter, and JavaScript API coverage for disabled and enabled profiles.
- Publish exact focused evidence and document the enabled-profile behavior.