WebAssembly bulk memory: decode and validate segments and instructions

Parent: #272 Depends on: #262 Roadmap: #263, #142, #134

Implement the pinned bulk-memory binary and validator surface before enabling execution.

Acceptance:

• Decode DataCount in its specified section order and validate its exact relationship to data indices.
• Decode active, passive, and declarative element segment forms plus active/passive data forms.
• Decode exact immediates for memory.init/data.drop/memory.copy/memory.fill/table.init/elem.drop/table.copy.
• Validate index spaces, element/reference types, memory/table compatibility, operand order, and stable malformed/disabled-feature diagnostics.
• Add focused binary and validator tests for every encoding family and rejection class.

Completed in d5eef86a. The IR and decoder cover DataCount in its specified order, all eight element forms, all data forms, and exact immediates for subopcodes 8-14. Validation covers segment/index/type/operand/DataCount rules with stable disabled and malformed diagnostics. Decoder evidence is 16/16 and the dedicated validator acceptance/rejection filter is 4/4; the combined Wasm checkpoint is 159/159, all with zero skips/failures/leaks.