Emit declarative element segments for ref.func references in WasmGC

[zacharywhitley]

Summary

The WebAssembly spec requires that functions referenced by ref.func outside of active/passive element segments must be declared in a declarative element segment (elem declare func ...). The WasmGC backend uses ref.func in heap initialization code (DynamicHub newInstance and clone function pointers) but never emits the required declarations.

This causes validation failures with strict validators:

• wasm-tools validate reports "undeclared function reference"
• wasmtime rejects modules assembled by wasm-tools parse
• Binaryen's wasm-as silently adds declarations, masking the issue

Changes

• Add declarativeFuncRefs set to WasmModule to track functions needing declarative element segments
• Add RefFuncCollector visitor in WasmPrinter that scans all function bodies, globals, and table initializers for ref.func instructions
• Emit (elem declare func ...) in the WAT output with all collected references

The fix is general — it handles any current or future ref.func usage automatically, not just the heap initialization paths.

Test plan

• Existing WasmGC tests pass (no behavioral change — Binaryen already handled this implicitly)
• wasm-tools validate --features all passes on WasmGC output
• wasm-tools parse successfully assembles WasmGC WAT to binary

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