fix: WAST validation improvements v3 — SIMD, GC refs, section ordering, const exprs

[avrabe]

Summary

Improves WAST test conformance pass rate from ~93% to ~94.6% (61,100 → 62,081 assertions passing out of ~65,624).

Changes by commit

1. RefType::Gc and SIMD types — Added GC reference types, SIMD instruction variants, and table64 support to foundation types
2. Compilation fixes — Resolved cascading compilation errors from the new types
3. SIMD opcode parsing — Added SIMD 0xFD prefix opcode parsing and GC reference type support in the parser
4. SIMD memory operations — Implemented v128.load/store/splat/lane operations in the engine
5. SIMD instruction dispatch — Added comprehensive SIMD instruction dispatch (comparison, arithmetic, conversion)
6. Test fixes — Fixed 4 test failures and reduced production panics
7. SIMD validation — Fixed SIMD unary/binary classification and memory64/table64 type checking
8. Encoded reference types — Handle 0x63/0x64 (ref/ref null) in table and element sections in decoder
9. Unreachable code validation — Improved alignment checking, const expr validation
10. SIMD lane/export/segment/tag validation — Additional validator checks
11. Type reference validation — Validate type indices are in bounds, GC reference compatibility in tables
12. Section ordering — Validate non-custom sections appear in spec order, reject duplicates
13. Const expr subtyping — Use is_subtype_of() for const expr end checks, proper ref.func/ref.null typing
14. ref.null LEB128 parsing — Use proper signed LEB128 for heap type in function bodies (not single byte)

Test results

• 62,081 assertions passing (up from ~61,100)
• 132 test files fully passing (up from ~125)
• 3,543 assertions still failing

What's NOT included (see follow-up issues)

• Unreachable code excess value detection (needs polymorphic stack tracking)
• GC structural subtyping validation
• Additional assert_malformed decoder improvements
• Runtime execution fixes (linking, call_indirect, exception handling)

[github-actions]

🔍 Build Diagnostics Report

Summary

Metric	Base Branch	This PR	Change
Errors	0	0	0
Warnings	2	2	0

🎯 Impact Analysis

Issues in Files You Modified

• 0 new errors introduced by your changes
• 0 new warnings introduced by your changes
• 0 total errors in modified files
• 0 total warnings in modified files
• 0 files you modified

Cascading Issues (Your Changes Breaking Other Files)

• 0 new errors in unchanged files
• 0 new warnings in unchanged files
• 0 unchanged files now affected

Note: "Cascading issues" are errors in files you didn't modify, caused by your changes (e.g., breaking API changes, dependency issues).

✅ No Issues Detected

Perfect! Your changes don't introduce any new errors or warnings, and don't break any existing code.

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🔧 To reproduce locally:

# Install cargo-kiln
cargo install --path cargo-kiln

# Analyze your changes
cargo-kiln build --output json --filter-severity error
cargo-kiln check --output json --filter-severity warning