WebAssembly Core 3: implement explicit exception heap references

Parent: #366 Related: #266, #142 Roadmap: #134

Core 3 explicit exception heap references are implemented without weakening the exact wg-3.0 profile.

Acceptance:

• Add the Core 3 exception and no-exception abstract heap types and binary encodings.
• Validate explicit exception-reference nullability/subtyping and catch-ref label results.
• Preserve typed exception payload and reference identity through catch_ref and catch_all_ref.
• Keep JavaScript WebAssembly.Exception behavior and GC roots exact.
• Pass every applicable command in the exact wg-3.0 exception directory.
• Add focused unit and bounded upstream-corpus CI evidence.

Delivered in e25cb50.

Completed on main in e25cb50. Evidence: 270/270 focused Wasm units with zero leaks; 98/100 exact Core 3 exception commands with zero failures and the two text-only malformed assertions explicitly N/A; 34/34 ref_null.wast; 17/17 corpus-harness tests. The shared pinned wasm-tools CI leg now runs both exact scopes.