fix(merge-batch-graphs): extend tested_by linker to cross-dir src/↔tests/ layouts (#302)

[tirth8205]

Summary

• Adds a basename-indexed Tier-B fallback to the path-convention tested_by linker so standard Python src/pkg/foo.py ↔ tests/test_foo.py (and JS/TS src/pkg/foo.ts ↔ __tests__/foo.test.ts) layouts actually produce supplemental edges instead of zero — fixes tested_by path-convention linker emits 0 supplemental edges on standard src/ ↔ tests/ layouts #302.
• Strictly additive: Tier A (the exact-path candidate list from production_candidates) runs first, so existing sibling, walkout, and mirrored pairings keep their precise semantics. Tier B only fires when Tier A misses.
• Scoring intent: same-dir > matching-tail mirror (tests/X/... ↔ src/X/...) > flat-tests anywhere under src//app//lib/ > non-src/ repo > weak last-resort. Tie-breakers: shortest path then lexicographic, for determinism.
• Pass-1 LLM-emitted (and inverted) edges still win — Tier B does not duplicate covered pairs.

Root cause

The original production_candidates enumerates candidate paths from the test's filename and dir alone, so for a flat tests/test_foo.py it can only generate tests/foo.py, foo.py, src/foo.py, app/foo.py, lib/foo.py. None of those reach src/pkg/foo.py. The fix indexes production files by basename so that lookup can find src/pkg/foo.py regardless of subdir depth, then scores each candidate against the test path to pick the most plausible pairing.

Test plan

• python3 -m unittest tests.skill.understand.test_merge_batch_graphs — all 84 tests pass (69 pre-existing + 15 new).
• New CrossDirBasenameFallbackTests class covers:
  • Python src/pkg/foo.py ↔ flat tests/test_foo.py (both test_* prefix and *_test suffix forms)
  • Python pkg/foo.py (no src/ prefix) ↔ tests/test_foo.py
  • Tier-A precedence preserved when both an exact-path candidate and a basename match exist
  • Deterministic tie-breakers when multiple production files share a basename (prefer src/ over scripts; alphabetical among equal-rank candidates)
  • Go same-dir _test.go regression coverage (Tier A still owns this case)
  • Go nested package paired against flat tests/ via Tier B
  • TS src/pkg/foo.ts ↔ __tests__/foo.test.ts and tests/foo.test.ts
  • TS .test.ts falling back to a .tsx production file via the extension family
  • Sibling src/foo.ts ↔ src/foo.test.ts regression
  • No-prod-counterpart path (must not invent edges)
  • Test-only nodes (must not link test ↔ test)
  • Pass-1 LLM-paired edges suppress duplicate Tier-B supplements

Closes #302