feat: selective imports and auto-import improvements

[klaidliadon]

Summary

• Auto-import prefers defining files over re-exporters — when multiple workspace files expose the same type, the LSP now suggests importing from the file that defines it locally, not one that re-exports it
• Narrowing diagnostics — warns when a full import brings in more types than are used, with a quick fix to convert to inline selective import syntax (import path.ridl + - Member)
• Unused import diagnostics — warns when an import contributes zero used types, with a quick fix to remove it
• Transitive re-import diagnostics — warns when a selective import pulls a type from a file that didn't define it, with a quick fix to import from the original source

Test plan

• Verify auto-import suggests the defining file when a type is re-exported by another file
• Verify narrowing warning appears for full imports where only a subset of types is used
• Verify no warning when all exported types are used
• Verify unused import warning when zero types from an import are referenced
• Verify narrowing quick fix converts block-form import to valid inline selective import
• Verify transitive re-import warning when selective import pulls from wrong source
• Verify transitive quick fix moves member to correct source file
• Run full test suite: go test ./...