docs(stammbaum): layout glossary + AC3 deferral SQL (#361)

@Elicit on PR #693: two doc gaps that block traceability on this PR.

1. docs/GLOSSARY.md: add a Stammbaum section with the layout vocabulary
   introduced by #689 and #361 — Stammbaum, seeded rank, sibling block,
   loose spouse, parented, anchor index, intra-family marriage, marriage
   dot, canonical fixture. Removes the Pending placeholder.

2. docs/adr/026: commit the AC3 reachability probe (the SQL that returned
   "0 of 942 unseeded persons match the predicate" in May 2026) directly
   into the ADR. A future architect re-evaluating the deferral can rerun
   it verbatim — reproducibility of the decision is itself a requirement.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/adr/026-stammbaum-layout-in-house.md

@@ -86,7 +86,27 @@ threshold, so `packBlocks.ts` is **not** yet warranted.
   with `generation`, P themselves was not) and belongs in the canonical import
   sheet rather than `buildLayout`. **Revisit trigger:** first canonical fixture
   containing a parented unseeded spouse — at which point this ADR is updated
-  in place or superseded by an ADR-027.
+  in place or superseded by an ADR-027. **Reproducible verification query**
+  (PostgreSQL — paste into a read-only psql session against
+  `familienarchiv_archive`):
+
+  ```sql
+  -- AC3 reachability probe. Returns one row per unseeded person who has at
+  -- least one parent edge whose parent IS seeded. A non-zero count means the
+  -- AC3 layout branch becomes reachable for that person and ADR-026 should
+  -- be revisited. Last run May 2026: 0 rows.
+  SELECT p.id, p.display_name
+  FROM persons p
+  WHERE p.generation IS NULL
+    AND EXISTS (
+        SELECT 1
+        FROM person_relationships r
+        JOIN persons parent ON parent.id = r.person_id
+        WHERE r.relation_type = 'PARENT_OF'
+          AND r.related_person_id = p.id
+          AND parent.generation IS NOT NULL
+    );
+  ```
 - **AC6 — Bundle-impact gate (≤ 40 kB gzipped on `/stammbaum`).** Moot under
   this ADR; reactivates only under ADR-027 (dagre adoption).
 - **AC7 — Visual regression at 320 / 768 / 1440.** `toHaveScreenshot()`