BridgeJS: Module-qualify internal ABI names to fix cross-module type collisions

[krodak]

Fixes swiftwasm#785.

Problem

When two Swift modules in a single BridgeJS build both export a @JS type with the same name (e.g. Point), the generated glue collides silently:

• duplicated @_expose(wasm, "bjs_Point_init") thunk names,
• duplicated swift_js_struct_lower_Point / swift_js_struct_lift_Point extern hooks and their JS-side bjs[...] handlers,
• colliding structHelpers.Point / enumHelpers.Point keys in the shared JS glue.

The JS handlers are plain assignments, so the last-linked module wins and every other module's values are decoded with the wrong field schema — silent data corruption rather than a build error.

Approach

Type identity in the generated glue is now module-aware, while the public JS surface stays flat:

1. Module-qualified payloads. Exported-type BridgeType payloads are minted as Module.[Nesting.]Name at skeleton-generation time. Cross-module references resolve through ExternalModuleIndex, which indexes by module-stripped path (so existing Point / Dep.Point references keep working) while carrying the defining module's qualified payload.
2. abiName derived from the qualified name by construction. abiName = swiftCallName.replacing(".", "_"), so the invariant JS key derived from payload == defining type's helper key holds structurally instead of by parallel derivations. All bjs_* thunks, swift_js_struct_lower/lift_* externs, and structHelpers/enumHelpers keys become Module_…-qualified.
3. Public surface unchanged. JS class names, exports keys, and TypeScript declarations are untouched — the re-record produced zero .d.ts diffs. Display names strip the module prefix.
4. Build-time diagnostic. The linker now rejects two different modules exporting the same public name at the same namespace path (classes, structs, non-namespace enums, top-level functions), with an error naming both modules. Namespace enums are exempt (cross-module namespace merging is intentional).

Protocol method hooks (wasm imports, already scoped per module by the import object) and TS-imported types keep their existing naming.

Testing

• sameNamedTypesAcrossModulesLinkWithDistinctABINames — links two skeletons that each define @JS struct Point plus same-named top-level functions; asserts distinct module-qualified externs and helper keys in the output (snapshot: SameNamedTypesAcrossModules.js/.d.ts).
• duplicatePublicNamesAcrossModulesProduceDiagnostic / duplicatePublicFunctionNamesAcrossModulesProduceDiagnostic — the new linker error.
• Full snapshot suite re-recorded (~150 fixtures churn from the abiName rename; internal names only).
• ./Utilities/bridge-js-generate.sh zero drift; npm run check:bridgejs-dts passes; WASM E2E (make unittest) 32/32 suites pass.

Notes for reviewers

• ABI names are internal and regenerated atomically on both sides of the bridge, but this is a rename of every generated symbol — anyone snapshot-diffing generated output downstream will see churn.
• @JS(namespace:) components no longer appear in type abiNames (module + Swift nesting path is used instead, which is unique per module and fixes a latent invariant violation for attribute-namespaced structs).
• Same-named classes/enums across modules in different namespaces still collide on public JS declarations (duplicate class X / XValues emissions). That's a pre-existing public-surface limitation, distinct from the ABI corruption fixed here — happy to file a follow-up issue.