Phase 1: multi-module region-imports (L13 positive-form) + cross-module differential fixtures

[hyperpolymath]

Follow-up carved out of #128 after its linear-ownership boundary half landed in #139 (merged). #128 delivered producer-side multi-module emission at parity with the verifier's existing cross-module coverage (extract_exports + verify_cross_module — the L10 linear import boundary). This issue tracks the remainder.

Remainder

• L13 positive-form / region-imports. Producer-side emission of the typedwasm.region-imports carrier for shared-region schema agreement across modules — the examples/02-multi-module.twasm "killer feature": one module export regions a schema, others import region … from it.
  • Blocked: the carrier is proposal 0003 [draft] (docs/proposals/0003-region-imports-carrier.adoc) with no verifier pass yet, so emission would be unverifiable today. Gate: proposal 0003 leaves draft + a verifier pass lands.
  • Field IDs must be shared-schema-stable across modules (per docs/developer/handoff-typed-access-sites-codegen.adoc).
• Cross-module differential fixture. Land a producer-emitted callee/caller .wasm pair under crates/typed-wasm-verify/tests/fixtures/c5_real/, exercised against verify_cross_module. (The in-crate crates/typed-wasm-codegen/tests/multimodule.rs is the parity oracle until then.)
• Wire examples/02-multi-module.twasm. Build it through the producer once the front-end → IR seam (Phase 1 / D1: codegen coverage — all 10 levels across all 6 .twasm examples #127) lands; today the multi-module emitters use hand-built in-Rust IR (like example 01).

Context

• Delivered: Phase 1 / D7: multi-module codegen at verifier parity #128 → PR Phase 1: multi-module codegen at verifier parity (#128) #139 — typed-wasm-codegen multi-module (multimodule_callee / multimodule_caller, 5 round-trip tests through verify_cross_module).
• Wire format: proposal 0003 (docs/proposals/0003-region-imports-carrier.adoc).
• Verifier cross-module reference: crates/typed-wasm-verify/src/cross.rs.

Part of Phase 1 (#49).