feat: resolve local Go package calls safely

[Helweg]

Summary

• resolve direct, unshadowed Go calls to one eligible same-package function in another indexed file
• keep selector calls, build-constrained files, incompatible tests, external packages, and ambiguous candidates unresolved
• refresh affected local Go package callers when source or package membership changes without re-embedding unchanged chunks

Validation

• npm run build && npm run typecheck && npm run lint && npm run test:run
• public Go call-graph, branch-index, PR-impact, and resolver tests

Safety

• resolution is bounded to a unique indexed function in the same eligible package directory