fix: bound Rust coverage dependency resolution

[cjorge-graphops]

Problem

Rust scans did not apply runtime/config exclusions while discovering Cargo packages, so ignored nested checkouts leaked into dependency resolution. Test coverage also rebuilt the full production-file index for every import and barrel candidate, turning a ~1,000-file project scan into a 20+ minute metadata-stat loop.

Fix

• discover Cargo manifests through the exclusion-aware source walker
• key the package-index cache by runtime exclusions
• build direct test mappings in one pass instead of production×test resolution
• reuse cached Rust file, workspace, and production indexes across import and barrel resolution
• match module candidates against lexical/resolved in-memory indexes without filesystem probing
• add regressions for excluded workspaces, one-pass mapping, and shared production-index reuse

Verification

• focused Rust/coverage suite: 103 passed
• real Autopilot V2 scan: all 13 phases complete; coverage falls from 20+ minutes to seconds
• full upstream suite: 5,795 passed, 151 skipped; 5 unrelated failures reproduce at upstream HEAD (three Bash unused-source cases and two review prompt cases)
• Ruff and diff checks pass on changed paths