practice restructure — per-problem jobs + ONE shared background-LLM pool (owner ruling 2026-09-29)

Rulings: each problem generation is its own visible dock job (the
extraction per-chunk pattern), and practice shares extraction's
semaphore — one process-wide 3-permit pool for ALL background LLM
jobs; total background LLM concurrency bounded at 3.

A. SHARED POOL: new src/agent/subagents/bg-llm-semaphore.ts —
   globalThis __lhLlmSlots, PERMIT_COUNT 3 (the ruling), the
   fresh-take/handed-off-permit deadlock discipline preserved for
   BOTH callers. extraction.ts's withExtractSlot is now a thin
   wrapper (job-row signal → withBgLlmSlot); __lhExtractSlots and
   __lhPracticeSlots both die. The partitioner now acquires a permit
   too (it's a background LLM job — it didn't before).

B. REGISTRY ROWS: the aggregate setId:generation / setId:grading rows
   are replaced by setId:partition (phase partitioning, label
   'Partitioning — title'), setId:gen:problemId (queued→generating,
   'Problem i/N — title'), setId:grade:problemId (queued→grading,
   'Grading i/N — title'). Each row carries its OWN abort controller
   and its OWN usage (one PracticeStepLog per row — the aggregate
   partitioner/generator/grader folding dies). Row key = feed id =
   abort key (the dock dup-key law).

C. ABORT SEMANTICS: abortPracticeRow(key) routes by shape — gen row:
   THAT problem only (error 'Aborted', siblings continue, set settles
   ready with one errored problem); partition row: the whole run;
   grade row: that grade drops (answer kept, re-submittable).
   abortPractice(setId) composes from row aborts + the synchronous
   persisted marks (a set-abort's verdict stands before the settles
   re-read it — the settle step keeps an existing error status).
   /api/activity/abort kind practice now takes the ROW key (charset
   admits ':').

D. FEED + DOCK: ActivityPractice/FeedPractice carry per-row identity
   (id = row key, problemIndex/problemTotal); dock blobs render per
   row, labels carry the i/N; the aggregate row is gone.

E. Boot sweep unchanged at set grain; the live-row check is now a
   key-prefix scan (any live row of the set protects it). The
   updatedAt heartbeat covers the new row scheme as-is.

Tests (20 in the pipeline suite): per-problem row shape/uniqueness/
i-N labels (partition row frozen behind a gated partitioner; per-row
usage ISOLATION pin — b's log never shows on c's row); abort-one-gen
(sibling completes, set ready with one errored problem); partition-
row abort; queued grade-row abort (dead-ticket path: answer kept,
re-submittable, sibling lands); shared-pool identity + contention
(practice queues behind extraction-style holders); registry/dock
feed mapping. Test isolation hardened: unique per-store set ids,
afterEach registry/lock/pool reset (a timed-out test can no longer
poison the rest).

architecture.md: jobs-pattern paragraph rewritten (per-problem rows,
shared pool, abort routing).

1149/1149, tsc 0, lint 0, build 0.

app/api/activity/abort/route.ts

@@ -30,7 +30,9 @@ export async function POST(req: Request) {
 	if ((kind !== "job" && kind !== "extraction" && kind !== "turn" && kind !== "practice") || typeof id !== "string" || id === "") {
 		return Response.json({ error: 'Body must be {kind: "job"|"extraction"|"turn"|"practice", id: string}' }, { status: 400 });
 	}
-	if (!/^[A-Za-z0-9_-]+$/.test(id)) {
+	// Practice row keys carry ":" (setId:gen:problemId) — the charset
+	// admits it for every kind (job/turn/extraction ids never use it).
+	if (!/^[A-Za-z0-9:_-]+$/.test(id)) {
 		return Response.json({ error: "Invalid id" }, { status: 400 });
 	}
 
@@ -50,10 +52,11 @@ export async function POST(req: Request) {
 	// kind === "extraction": true cancellation; idempotent (nothing in
 	// flight is success — same dock outcome)
 	if (kind === "practice") {
-		// practice slice 4 (2026-09-29): abort BOTH of a set's live runs
-		// (generation + grading) and mark the persisted "Aborted" state —
-		// idempotent like extraction (nothing in flight is success).
-		practice.abortPractice(id);
+		// practice restructure (owner, 2026-09-29): id is the ROW key —
+		// abort routes by shape (a gen row kills THAT problem only; the
+		// partition row ends the run; a grade row drops that grade).
+		// Idempotent like extraction (nothing in flight is success).
+		practice.abortPracticeRow(id);
 		return Response.json({ ok: true });
 	}
 	abortExtraction(id);