fix(decoder): GC ref type decoding and custom page sizes support

[avrabe]

Summary

• Parse multi-byte GC reference types (0x63/0x64 + heap type) in local variable declarations and global import types — previously only single-byte value types were handled, causing GC modules to fail decoding
• Accept custom-page-sizes proposal flag (bit 3) in memory limits, validate page size is power of 2 and <= 65536
• Add page_size: Option<u32> field to MemoryType across all construction sites

Test plan

• custom-page-sizes.wast: 0/45 → 33/45 passing
• struct.wast: modules now decode successfully (6 modules load)
• ref_eq.wast: modules decode past decoder stage
• cargo check passes

Trace: skip

🤖 Generated with Claude Code

[github-actions]

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Summary

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

🎯 Impact Analysis

Issues in Files You Modified

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• 0 total errors in modified files
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• 0 files you modified

Cascading Issues (Your Changes Breaking Other Files)

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• 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