docs: handoff 21 — ADR-0028 complete

ADR-0028 (query plans / `explain`) is fully implemented; the
handoff-16 design trio (ADR-0026 / 0027 / 0028) is now closed.

- handoff-21: session summary, the two deliberate deviations
  from handoff-20's plan, test coverage, open clusters.
- requirements.md: QA1 / QA2 ticked.
- CLAUDE.md: the `EXPLAIN QUERY PLAN` deferred-items line
  updated to "implemented per ADR-0028".

docs/requirements.md

@@ -371,18 +371,17 @@ handoff-14 cleanup; 449 after B2/C2.)
 
 ## Query analysis
 
-- [ ] **QA1** `EXPLAIN QUERY PLAN` is run on demand for queries;
+- [x] **QA1** `EXPLAIN QUERY PLAN` is run on demand for queries;
   output is rendered as an annotated tree highlighting full
   scans, index use, and join order.
-  *(Designed in ADR-0028: the `explain` prefix over
+  *(Implemented per ADR-0028: the `explain` prefix over
   `show data` / `update` / `delete`, with a span-styled plan
-  tree. The pedagogical payoff — a plan that flips between a
-  full scan and an index search — needs a filtered query
-  (`show data … where`, designed in ADR-0026) plus indexes
-  (ADR-0025, done). Implementation pending.)*
-- [ ] **QA2** Plan rendering specifics — tree layout, annotation
-  taxonomy, colour scheme. Designed in ADR-0028 (§3–§6);
-  implementation pending.
+  tree. `EXPLAIN QUERY PLAN` never executes, so explaining a
+  destructive `update` / `delete` is safe.)*
+- [x] **QA2** Plan rendering specifics — tree layout, annotation
+  taxonomy, colour scheme. Implemented per ADR-0028 (§3–§6):
+  a box-drawing tree, the substring-pattern taxonomy, and the
+  `OutputLine` styled-runs mechanism.
 
 ## Hints, help, errors