Transitive external imports can be overwritten by simple-name collisions

[danfma]

Summary

_externalImportMap is keyed by simple name and is overwritten while discovering [Import] declarations from referenced assemblies. This makes external import resolution non-deterministic when two libraries expose bindings under the same TS name.

Evidence

• MetaSharp.Compiler.TypeScript/Transformation/TypeTransformer.cs:323 does _externalImportMap[type.Name] = entry.
• MetaSharp.Compiler.TypeScript/Transformation/TypeTransformer.cs:327 does _externalImportMap[tsName] = entry.
• The comment immediately above says this phase should only add entries and should not overwrite local ones, but the implementation overwrites unconditionally.

Risk

A transitive dependency can silently replace a local external import mapping. The result depends on Compilation.References ordering, which makes the behavior hard to predict and debug.

Recommendation

Replace simple-name indexing with symbol-qualified or origin-qualified resolution, or at minimum detect collisions and emit a diagnostic. Local entries should explicitly win over transitive ones.

Missing tests

• Two referenced assemblies with [Import] for the same simple name.
• One local assembly and one referenced assembly defining the same external name.
• A collision involving AsDefault = true and a named import.