WebAssembly memory64: pinned corpus, binary IR, and validation

Parent: #267 Depends on: #262 Roadmap: #142, #134

Pin the exact memory64 proposal surface and implement its binary and validation boundary.

Acceptance:

• Check in an exact revision, file/command inventory, value/limit encodings, instruction immediates, and dependency declaration.
• Decode 64-bit memory limits, memory types, data/element offsets, and memory instructions behind the memory64 gate.
• Validate address/result widths, memory-index selection, alignments, limits, and disabled-feature diagnostics exactly.
• Add malformed/invalid, failure-atomicity, and allocation-failure witnesses plus drift verification.

Delivered in 90d21bf6, b692aeeb, 3d0418d4, dda96c72, and 004513b7.

Runtime/JavaScript API behavior remains intentionally separate in #297; terminal proposal scoring and evidence remain in #300.

Completed and pushed to main. Evidence: exact memory64 pin/inventory (23 files, 13,918 commands, 824 memory64 declarations, 151 table64 declarations); normative u64 limits/memarg offsets; memory64/table64 IR and validation across scalar, SIMD, atomic, bulk, active-segment, indirect-call, size/grow paths; deterministic feature/malformed diagnostics; decoder allocation-failure rollback coverage. Verification: memory64 filter 7/7, decoder filter 24/24, validator filter 53/53, zero leaks; feature registry verifier green. README and docs explicitly separate this boundary from #297 execution/API and #300 terminal scoring.