docs(plan): per-sale expiry tracking design — batch registry + candidate matching

Grilled 2026-07-16 with the user; full decision record in
docs/expiry-tracking-plan.md. Core reframe: expiry is captured once per
batch at DO intake (staff-typed on the stock-entry confirmation page, from
the physical packs), so the cashier scan only MATCHES OCR fragments against
the 1-3 known in-stock batch dates instead of free-reading damaged
dot-matrix prints (proven model-capability ceiling, 2026-07-15). Fallback:
auto-FEFO + 'inferred' flag, zero cashier interaction. No cloud, ever.

- docs/expiry-tracking-plan.md: architecture, matching algorithm spec
  (resolveExpiryFromEvidence), schema/API deltas, phases 1-3, testing plan
- backend plans §13 (13.1-13.4): matcher util + offline tuning, route
  wiring + expiry_source provenance, multi-frame union, dot-matrix
  recognizer fine-tune
- root plans §10 (10.1-10.3): cashier fast path, inferred badge +
  end-of-day review, burst capture for mounted camera
- stock-feature-plan.md: extension note (batch dropdown becomes the
  manual-override path)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q8TumxFDnyVnfsR3mxPXfX

docs/stock-feature-plan.md

@@ -11,6 +11,14 @@ backlog entries in root [`plans/next-enhancements.md`](../plans/next-enhancement
 codebase yet — this doc is the design record to build from when the tasks below are
 picked up via `n`/`next`.
 
+**Extended 2026-07-16 by [`expiry-tracking-plan.md`](expiry-tracking-plan.md)**
+(backend §13 / root §10): the Product Scan batch *selection* described in §5/§10
+below becomes an automated candidate-matching resolution at the cashier
+(fragment-match OCR evidence against the in-stock batches this plan registers;
+auto-FEFO + `inferred` flag as fallback). The dropdown UX below survives as the
+manual-override path. Schema, decrement semantics, and everything else in this
+doc are unchanged.
+
 ## Context
 
 The app currently tracks Delivery Order (DO) documents and a "Product Scan"