[Detection] Audit barrel-file / re-export resolution (A3) #75

Summary

Verify and harden the barrel-file (index.ts re-export) resolution path. Build a fixture matrix covering all re-export shapes and fix the ones that don't resolve.

Why

TS projects heavily use barrels. If a barrel breaks the import chain, the scanner stops at the barrel boundary and silently drops every downstream call.

Current state

src/ast/cross-file-resolver.ts header comment lists "barrel re-exports" as pattern #4. Unclear which exact re-export shapes work — needs a fixture-driven audit.

What to do

1. Build fixtures for: export *, export { x }, export { x as y }, export { default }, nested barrels, mixed barrels.
2. Run the resolver against each. Document gaps.
3. Fix the gaps.
4. Check performance on a 1000-file repo to ensure no resolution blow-up.

Acceptance criteria

• All five re-export shapes resolve correctly
• Nested barrels (2 levels) resolve
• Imports pointing to non-existent symbols fail gracefully
• Scan time on a 1000-file repo not impacted measurably

Reference

Full design: https://github.com/recost-dev/extension/blob/main/docs/accuracy/detection.md#a3-barrel-file--re-export-resolution-audit