Implements the WebAssembly Core 2.0 binary-format decoder as the foundation for Phase 22. Zero external dependencies, 100% safe Rust (forbid(unsafe_code)). - Width-bounded LEB128 readers (32/33/64 bit) with overflow/overlong detection, IEEE-754 LE float readers, length-prefixed UTF-8 names. - Full preamble + section dispatch (ids 0..=12, custom preserved verbatim, duplicates of non-custom sections rejected). - All numeric, parametric, variable, table, memory, control, and reference opcodes from MVP plus post-MVP sign-extension, saturating trunc, reference types, bulk memory, and multi-value blocktypes. - Structured ParseError variants carry byte offsets and diagnostics. - 33 tests covering LEB128 edge cases, preamble, each section type via hand-authored fixtures, instruction decoding, and a real Rust-compiled wasm32-unknown-unknown module fixture. Closes the "WebAssembly binary module parser" Phase 22 issue.