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;

src/core/ports.ts

@@ -21,6 +21,7 @@ import type {
 	ExtractionRunUsage,
 	PlanTree,
 	Position,
+	ProblemSet,
 	CoverageEntry,
 	SessionCheckpoint,
 	TurnUsage,
@@ -157,6 +158,33 @@ export interface JobStore {
 	deleteJobsForChat(chatId: string): number;
 }
 
+// ─── PracticeStore — problem-set documents (data/practice) ──────────────────
+
+/** Everything a new set needs except the store-assigned "ps_" id. */
+export type CreateProblemSetInit = Omit<ProblemSet, "id">;
+
+/**
+ * The practice module's persistence: one JSON document per problem set
+ * (src/store/practice.ts adapter). Sets are bounded, SELF-CONTAINED
+ * snapshots — propositions + chains frozen at creation (contracts/
+ * practice.ts) — so reads never join the live graph. Replace-whole
+ * writes like settings.json, one-file-per-record like data/jobs.
+ * Concurrent same-set writes are the pipeline's serialization duty
+ * (slice 4), not the store's — this seam owns file integrity only.
+ */
+export interface PracticeStore {
+	/** Persist a new set; the store assigns the id ("ps_" + ts36 + rand). */
+	createSet(init: CreateProblemSetInit): ProblemSet;
+	/** The full set document, or null when unknown id / corrupt file. */
+	getSet(id: string): ProblemSet | null;
+	/** Every set, createdAt descending (the practice page's list view). */
+	listSets(): ProblemSet[];
+	/** Persist the full set document (replace-whole). */
+	saveSet(set: ProblemSet): void;
+	/** Delete a set file; false when unknown id. */
+	deleteSet(id: string): boolean;
+}
+
 // ─── SettingsStore — THE user settings record (settings-storage wave, 2026-09-19) ──
 
 /**