core: Support arbitrary nesting depth in struct field access

Replaces single-level struct field access with unified architecture supporting:
- Arbitrary nesting depth (event.params.data.1.inner.0)
- Mixed numeric/named field access (event.params.asset.addr)
- Comprehensive error context with declarative call labels

Key changes:
- Replace ParamField with unified StructField(Word, Vec<FieldAccess>)
- Add FieldAccess enum supporting Index(usize) and Name(Word)
- Implement extract_nested_struct_field() for arbitrary depth traversal
- Update parser to handle unlimited nesting with mixed access patterns
- Add extensive test coverage for all access patterns

Named field access parsing is complete; extraction shows "not yet supported"
placeholder pending ABI integration for name-to-index resolution.

Addresses GitHub issue #6059.

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: fubhy