docs: add ADR-0028 — query plans (EXPLAIN QUERY PLAN)

The QA1/QA2 design: an `explain` prefix command over
`show data` / `update` / `delete` that runs
EXPLAIN QUERY PLAN (without executing the statement) and
renders the result as an annotated tree. Plan steps keep
the engine's own wording; an annotation taxonomy marks
full scans, index use, and the automatic-index "you
should add an index here" case. Introduces a general
styled-output-line mechanism — an OutputLine may carry
per-span styling — realising the per-span theming
ADR-0016 deferred; the plan renderer is its first
consumer. The explained SQL is shown above the tree as
standard, copy-pasteable SQL.

- docs/adr/0028-query-plans.md — the ADR.
- docs/adr/README.md — index entry.
- docs/requirements.md — QA2 [~] -> [ ]; QA1 note
  reconciled (designed in ADR-0028).

docs/adr/README.md

@@ -33,3 +33,4 @@ This directory contains the project's ADRs, recorded per
 - [ADR-0025 — Indexes](0025-indexes.md) — **Accepted**, `add index` / `drop index`, persistence, rebuild-table preservation, and items-list display (`C3` index portion + `S2`)
 - [ADR-0026 — Complex WHERE expressions](0026-complex-where-expressions.md) — **Accepted**, stratified recursive expression grammar (`AND`/`OR`/`NOT`, comparisons, `LIKE`, `IS NULL`, `IN`, `BETWEEN`) for `update` / `delete` / `show data` filters; `show data` gains `where` + `limit`; adds the `Subgrammar` node and a recursive `Expr` AST (`C5a`)
 - [ADR-0027 — Input-field validity indicator](0027-input-validity-indicator.md) — **Accepted**, a debounced `[ERR]` / `[WRN]` marker at the input row's right edge, backed by a walker diagnostics-severity model (parse-outcome + schema-existence); advisory, never blocks submission (`S6`)
+- [ADR-0028 — Query plans (`EXPLAIN QUERY PLAN`)](0028-query-plans.md) — **Accepted**, an `explain` prefix command over `show data` / `update` / `delete`; an annotated, span-styled plan tree; introduces the `OutputLine` styled-runs mechanism (ADR-0016's deferred per-span styling) (`QA1` / `QA2`)