docs: the opt-link question has an answer and a stated coverage limit

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PMRJjbxLqZtsb5Vb7KunPE

docs/re/BACKLOG.md

@@ -199,7 +199,16 @@ See [`structures/ui-composable-bundles.md`](structures/ui-composable-bundles.md)
   See [`structures/ui-rat-layout.md`](structures/ui-rat-layout.md).
 * ❔ **What makes a bundle a screen rather than a fragment.** `is_composable`
   admits 1 786 extra bundles, mostly 2–5-element button+glow fragments.
-* ❔ **What `opt ` links.** Refuted as focus; unexplained otherwise.
+* 🟡 **What `opt ` links — a record→record reference** (2026-08-24, measured
+  disc-wide and asserted). All **1 467** links reachable from a declaration table
+  resolve to a **RATC child of their own bundle**, all are `.rat → .rat`, none
+  dangle, none self-link. **1 076 (73 %)** are the `<stem>f` focus pattern; the
+  rest are **chains** between effect records (`px_bunk_eff01 → pjex_eff →
+  pjex_eff07`), which is also why only 227 targets are declared elements — the
+  middle of a chain is, the end is not. So focus is the commonest *use*, not the
+  meaning. ⚠️ Coverage: 18 718 raw `opt ` tags exist against 1 467 classified —
+  `opt_link` reads the first tag of a declared element's record, so ~92 % of
+  occurrences sit deeper in the chains and are untested.
 
 The investigation that got here follows, kept in full because most of it is
 refutations that were worth the cost.