practice slice 2 — store, paths, self-contained sets

Contract amendment (planning ruling): ProblemSet gains propositions
(nodeId → {name, statement}) and chains (the frozen derivation) — sets
are documents of what WAS selected; grading/dedup/coverage never touch
the live graph. config.ts PATHS.practice → data/practice/. PracticeStore
port (createSet assigns ps_<ts36>_<rand>, getSet, listSets createdAt-desc,
saveSet replace-whole, deleteSet). src/store/practice.ts: one JSON per
set, hand-written isProblemSet read guard (types-only; corrupt/missing →
null/skipped, never thrown; saveSet refuses invalid records). Wired in
src/server/stores.ts. 10-test suite: round-trip, corrupt-tolerance,
missing-dir, validator garbage-reject + optional-tolerate + verdict
union, ps_ id shape.

src/core/contracts/practice.ts

@@ -128,6 +128,16 @@ export interface ProblemSet {
 	 *  decomposition derives from these + the follows-edges among them at
 	 *  creation time. */
 	nodeIds: string[];
+	/** Snapshot of every selected proposition at creation — nodeId →
+	 *  {name, statement}. The set is SELF-CONTAINED (planning ruling,
+	 *  2026-09-29): rendering, generation, grading, and dedup never
+	 *  re-read the live graph, which keeps moving (extraction continues,
+	 *  nodes get deleted); a set is a document of what WAS selected. */
+	propositions: Record<string, { name: string; statement: string }>;
+	/** The frozen chain derivation (chainsForSelection at creation time).
+	 *  Coverage checks and the problems' chain tags resolve against THIS,
+	 *  never against live edges. */
+	chains: ReasoningChain[];
 	status: ProblemSetStatus;
 	/** Set when the partition itself (or the run as a whole) failed. */
 	error?: string;