docs(changeset): state which picker exit moved instead of claiming all three held

The diff deletes `if (!formField) return undefined;`, so "the three exits are
byte-for-byte unchanged" was inaccurate in text that ships to CHANGELOG.md.
Two exits are verbatim untouched; the third is deleted, and deleting it is the
fix — outside the declared position it was the silent exit, inside it it is
unreachable by construction.

Co-authored-by: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017ef78bLdybu3AffehKkhfk

Author: claude

.changeset/16403-picker-reader-position-guard.md

@@ -7,5 +7,5 @@
 `bindAncestors` walked out through a filter's ancestors and matched `if (key === PUBLIC_PICKER_KEY)` on the property NAME. `walkAuthoredFilters`/`scanForFilters` recognise a filter by key at ANY depth on all eight scanned collections, so that reader was reachable from any node named `publicPicker`, anywhere. Its unresolvable exit is `undefined` — no bound object, so arm 2's field-type oracle answers `false` for every key and the subtree is judged by nobody.
 
 - **No live defect today**: `publicPicker` is declared exactly once as a schema key, on `FormFieldBaseSchema` (`packages/spec/src/ui/view.zod.ts`), and there the reader is correct. What changed is the failure mode the day a second schema declares the same name: it would have inherited this branch silently. Under-reporting is this rule's only permitted failure direction, so the hole would never have VIOLATED that invariant — it would have quietly spent it, where no test asking "was the invariant violated?" could see it.
-- **The guard is on the entry, not the exits**: the branch now requires the enclosing ancestor to be a form field (`field`, required on `FormFieldBaseSchema`) — the same read the branch already had to make one line later, so no new coupling between the lint package and the form-view schema. The three exits `#16106`'s review pinned (the `picker.object` override, the `reference` resolution, and the `undefined` no-fall-through) are byte-for-byte unchanged.
+- **The guard is on the entry, not the exits**: the branch now requires the enclosing ancestor to be a form field (`field`, required on `FormFieldBaseSchema`) — the same read the branch already had to make one line later, so no new coupling between the lint package and the form-view schema. Two of the three exits `#16106`'s review pinned are verbatim untouched: the `picker.object` override (`if (override) return override;`) and both `undefined` legs of the `reference` resolution (`if (!formObject) return undefined;` and the `verdict?.kind === 'ok' ? … : undefined` tail). The third — `!formField` returning `undefined` — is DELETED, and deleting it IS the fix: outside the declared position that line was the silent exit this card is about, while inside the declared position it is unreachable by construction (the guard holding means `formField` is truthy). So the behaviour P3's QUIET pin holds did not move.
 - **The `#16106` B1 false refusal stays closed**, measured: a form field's picker filter over a referenced `select` column that shares its name with a parent `date` column still reports nothing, and the positive control — the same filter where the REFERENCED object declares the field as a `date` — still reports at `views[0].sections[0].fields[0].publicPicker.filter[0].value`.