fix(wast): comprehensive validator improvements for block/br conformance

- Fix block type parsing to handle type indices (not just value types)
- Add memory load/store stack manipulation for proper type checking
- Add f32/f64 comparison and binary operations to validator
- Update pop_type for frame boundary checking in unreachable code
- Results: block.wast 154→220, br.wast 20→90, i32.wast 460/460 stable

Author: avrabe