Discover list instantiations from bare literals in wasm-gc codegen

[jasisz]

The wasm-gc backend rejected any program whose only List<T> instantiation appears as a bare list literal — Console.print("{[1, 2, 3]}") in a Unit fn failed with List literal: cannot resolve list instantiation (got `List<Int>`). Surfaced by @n1bor while testing #764.

Two independent gaps in type discovery:

• a list literal's own instantiation was never registered — discovery walked fn signatures, let annotations, and record fields, so the emitter's canonical resolution succeeded but the registry lookup missed;
• the expression walkers never traversed string interpolations (or map literals), so nothing inside "{...}" was visible to discovery at all.

The body walk now carries Spanned nodes so the checker's type stamps are readable; list literals register their stamped type, behind a structural concreteness guard, with a first-element fallback mirroring the emitter's hint chain. All three collectors (lists, options, vectors) got the missing traversal arms, and the expression matches are now exhaustive, so a future HIR variant fails the build instead of silently skipping discovery.

Three regressions in wasm_gc_codegen_regression: interpolated bare literal, unannotated binding, nested literal — each compiled and validated with wasmparser.

wasm_gc_codegen_regression 5/5, wasm_gc_spec 37/37, cargo fmt --check clean, clippy clean with wasm features.