fix(validator): implement GC structural subtyping validation

[avrabe]

Problem

The validator lacks GC structural subtyping, causing:

• 21 assert_invalid failures for "sub type" validation
• 8 assert_invalid failures for "unknown type" in GC instructions
• ~20 module validation failures (false positives) in type-subtyping.wast, type-equivalence.wast, type-rec.wast

What's needed

1. Sub type validation: When a type uses (sub $parent ...), validate that:

   • The parent type index exists
   • The child type is structurally compatible with the parent
   • Function subtypes have contravariant params and covariant results

2. Recursive type groups: (rec ...) groups define mutually recursive types. Type indices within a rec group can reference each other.

3. Type equivalence for GC: Two types are equivalent if they have the same structure (iso-recursive interpretation). The validator must use structural comparison for type-indexed operations like call_indirect.

Impact

• ~49 directly failing assertions
• Additional cascading failures from false module validation rejections

Files

• kiln-build-core/src/wast_validator.rs — needs new validate_subtypes() function
• kiln-format/src/module.rs — may need sub/rec type information in Module struct
• kiln-decoder/src/streaming_decoder.rs — may need to decode sub/rec type declarations