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.

src/core/contracts/practice.ts

@@ -163,6 +163,12 @@ export interface ProblemSet {
 	 *  default. */
 	title: string;
 	createdAt: string;
+	/** Bumped on every store write — the live-run heartbeat the boot
+	 *  sweep's grace window reads (a live run saves at every phase or
+	 *  problem transition, so it is structurally never 120s stale).
+	 *  Absent on legacy rows (validator tolerates; readers fall back
+	 *  to createdAt). */
+	updatedAt?: string;
 	params: PracticeParams;
 	/** The selected node ids — the raw selection record. The chain
 	 *  decomposition derives from these + the follows-edges among them at