practice bug fix — the boot sweep stamped a LIVE run (owner live test 2026-09-29)

Owner report: created a set, 'Interrupted (server restarted)' showed
on list + set views, generation proceeded and overwrote it. The
sweep's 'generating + no registry row' test had a false-positive
mode: it ran at EVERY pipeline instantiation, and an instance could
evaluate against a registry whose fresh row it couldn't observe.

Three guards, per ruling:

1. HEARTBEAT: ProblemSet.updatedAt? (contract + doc) — bumped on
   every store write; createSet stamps it = createdAt; saveSet
   re-stamps (validator tolerates absence for legacy rows).

2. ONCE PER PROCESS: the sweep guards on globalThis
   .__lhPracticeSweepDone — no instance re-evaluates after the
   process's first sweep.

3. GRACE WINDOW: only sets whose heartbeat (updatedAt ?? createdAt)
   is older than 120s are stamped. A live run saves at every phase/
   problem transition, so it is structurally unreachable; a true
   orphan is minutes stale at next boot. One [practice <id>] log
   line per stamped set (the extraction/jobs console.error idiom).

Tests: stale orphan stamped; FRESH generating set untouched (grace);
once-guard (second factory instantiation never re-sweeps); live-row
protection re-armed via resetSweep() test seam; store pins for
updatedAt birth-stamp + re-stamp + legacy tolerance. Architecture.md
sweep sentence updated (once-per-process + grace semantics).

1142/1142, tsc 0, lint 0, build 0.

tests/practice-store.test.ts

@@ -42,6 +42,26 @@ const init = (over: Partial<Omit<ProblemSet, "id">> = {}): Omit<ProblemSet, "id"
 
 // ─── round-trip ──────────────────────────────────────────────────────────────
 
+describe("updatedAt — the live-run heartbeat", () => {
+	it("createSet stamps updatedAt = createdAt; saveSet re-stamps on every write", () => {
+		const set = practiceStore.createSet(init());
+		assert.equal(set.updatedAt, set.createdAt, "birth stamp");
+		const before = new Date().toISOString();
+		const edited = { ...set, title: "Renamed" };
+		practiceStore.saveSet(edited);
+		const read = practiceStore.getSet(set.id)!
+		assert.ok(read.updatedAt !== undefined && read.updatedAt >= before, "every write re-stamps");
+		assert.notEqual(read.title, set.title);
+	});
+
+	it("legacy rows without updatedAt still validate (readers fall back to createdAt)", () => {
+		const set = practiceStore.createSet(init());
+		const bare = { ...set } as Partial<typeof set>;
+		delete bare.updatedAt;
+		assert.ok(isProblemSet({ ...bare, id: set.id }));
+	});
+});
+
 describe("the practice store", () => {
 	it("createSet assigns a ps_ id, mkdirs, and round-trips through getSet", () => {
 		const set = practiceStore.createSet(init());