feat (sema): add module import resolution

[ofabiodev]

Summary

Adds T10 module and local import resolution to the semantic analysis layer.

Changes

• Resolve imports using path.puff and path/main.puff candidates
• Support default prefixes, explicit aliases, and namespaced imported symbols
• Add import diagnostics, path validation, and multi-file integration fixtures

Testing

make test
make build

cd src
go vet ./...
go test ./internal/sema -cover