Add wit-bindgen fixtures for remaining type families

[avrabe]

Context

Issue #1 introduced wit-bindgen composed-runner components as test cases. We now have 4 fixtures passing with full runtime verification (numbers, strings, lists, records).

Remaining type families

The wit-bindgen test suite covers additional type families that would exercise more adapter code paths:

• variants — variant, enum, option<T>, result<T, E>
• flags — bitflags types
• resources — handle types (may require resource table support)
• nested records — deeply nested record types with inner pointers
• multi-return — functions returning multiple values

How to generate

From the wit-bindgen source tree:

wit-bindgen test --languages rust --artifacts artifacts \
  --rust-wit-bindgen-path crates/guest-rust/ tests/runtime

Fixtures end up in artifacts/<name>/composed-runner.rs-test.rs.wasm.

Test structure

Follow the existing pattern in meld-core/tests/wit_bindgen_runtime.rs:

1. Fusion as CoreModule — validate with wasmparser
2. Fusion as Component — validate
3. Runtime execution — fuse, load in wasmtime with WASI, call run()

ref #1