fix: structural ref resolution for record-using interfaces

[colinrozzi]

Summary

Handlers that declare records inside an interface name { ... } block (e.g. theater-handler-podman) could not agree on an interface hash with their actors. Two root causes:

1. Handler side hashed Type::Ref(path) nominally (sha256("ref:" + path_str)), while the actor side hashed records structurally. Even when the actor-side type defs were in scope, the two sides produced different bytes.
2. Actor-side parser rejected records inside imports { iface { ... } } blocks, so actors couldn't declare matching type defs at all — the only escape hatch was placing records at top level of the .types file, which still mismatched (see point 1).

This unblocks agentry-actor and any future actor calling a record-using handler.

Changes

• src/metadata.rs: new hash_type_in(ty, types) resolves Type::Ref against in-scope typedefs and hashes the underlying record/variant structurally. Cycle-safe via a name stack returning HASH_SELF_REF. hash_type becomes a thin wrapper over hash_type_in(ty, &[]). compute_interface_hash threads arena.types through.
• src/interface_impl.rs: removed the duplicate ref-hashing path in type_to_hash (it shadowed the metadata.rs logic and would have diverged again). FuncSignature now has hash_in(types); the old hash() delegates with no types in scope. InterfaceImpl carries a types field, and from_pact captures pact.types (plus type-style exports). This is the path theater actually uses.
• crates/pack-guest-macros/src/lib.rs: parse_func_sigs_into accepts record/variant/enum/flags/type declarations at the top of an interface block. Declarations are scoped locally so sibling interfaces don't share resolution.
• crates/pack-guest-macros/src/metadata.rs: added an explanatory comment about the result<_, E> → Bool/String convention shared with pact.rs::parse_result.

Tests

• 4 new handler-side unit tests in src/metadata.rs: ref resolves structurally, unresolved ref falls back to path hash, recursive types terminate cleanly, compute_interface_hash matches a hand-computed structural hash.
• 4 new actor-side unit tests in pack-guest-macros: parser accepts records inside imports/interface blocks, scopes don't leak between sibling interfaces, top-level typedefs reach into blocks, result<_, E> convention.
• 1 cross-crate convergence test in tests/handler_actor_hash_convergence.rs using the actual podman.pact shape: parse_pact + InterfaceImpl::from_pact produces a byte-identical hash to packr-abi primitives applied directly. This is the test that would have failed before the fix and catches future regressions.

Test plan

• cargo test --workspace — all 252 tests pass
• Convergence test (handler_actor_hash_convergence) explicitly mirrors the user-reported theater-handler-podman scenario

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