perf: Cache or narrow TestSuiteGate test discovery (avoid full rglob per verify)

[omsherikar]

Scope

refactron/verification/checks/test_gate.py — _find_relevant_tests

Problem

Each verify() call runs search_root.rglob("*.py"), then for every candidate test file reads and parses the full source to see if it imports the module. On large projects this is O(number of Python files × number of autofix verifications).

Suggested direction

• Prefer conventional test roots (tests/, test/) when present; exclude .venv, node_modules, etc.
• Optional: cache a mapping module_stem → set[Path] on the gate instance or engine keyed by project_root mtime / file list hash; invalidate when project layout changes.
• Optional: static reverse import index built once per run.

Acceptance

• Verification latency scales with relevant tests, not entire repo size, on representative layouts.