fix(decoder): SIMD lane byte range and GC code body parsing (#150)

[avrabe]

Summary

• SIMD extract/replace lane range: Subops 14-20 (swizzle, splats) have NO immediates but were consuming a lane byte. Fixed range to 21-34 for actual extract/replace lane ops.

Impact

Baseline: 65,113 pass / 529 fail (after PR #170)
After: 65,157 pass / 485 fail (+44 assertions fixed)

Cumulative from original baseline: 64,068 to 65,157 (+1,089 assertions, -69% failures)

Test plan

• simd_splat.wast: 185/185 pass (was 141/185)
• simd_conversions.wast: 282/282 pass
• simd_i8x16_arith: 1001/1001 pass
• CI passes

[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