feat(recall): visualization layer — diff past approaches instead of listing them

[QodeXcli]

P2 — recall visualization. recall_approach answers "how did we solve auth before?" with a visual comparison instead of a flat list:

★ Best match  [🎯 task · 2d ago · 38% match]
   add jwt auth login middleware to the api endpoints
   files: src/auth/jwt.ts, src/auth/middleware.ts

How the other attempts DIFFERED from it:

2. [🎯 task · 3w ago]  add auth login with refresh token rotation flow
   + this one adds: refresh, token, rotation
   − it lacks: jwt, middleware, api
   files (+ = not in best match): +src/auth/refresh.ts, src/auth/jwt.ts

Stable core across all 3: auth, login — this is how you consistently approach it here.

Architecture (as planned)

rankApproaches (recency + MMR + stemming — already shipped) → pure visualization layer → markdown with diff blocks + file links + score.

• approach-diff.ts (PURE): termDiff — vocabulary diff compared by stem (paginate↔pagination = shared) but displayed as surface words; lineDiff — LCS unified ± block, auto-selected when both texts are genuinely multi-line (worklog entries); commonCore — the stable terms across every attempt; renderApproachDiffs — the markdown renderer, incl. +file flags for files an alternative touched that the best match didn't
• Tool visualizes top-4, lists the rest compactly — nothing hidden
• 12 new PURE tests (stem-shared vocab, LCS edges, file provenance flags, single/empty/identical fallbacks)

Full suite 1500 green, tsc clean.