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/server/stores.ts

@@ -9,6 +9,7 @@
 import { chatStore } from "../store/chats.ts";
 import { graphStore } from "../store/graph.ts";
 import { jobStore } from "../store/jobs.ts";
+import { practiceStore } from "../store/practice.ts";
 import { settingsStore } from "../store/settings.ts";
 import { resolveSettings, type EffectiveSettings } from "../core/settings-resolution.ts";
 /** The wired adapters. Satisfies core/graph.ts's GraphDeps structurally. */
@@ -16,6 +17,7 @@ export const stores = {
 	chatStore,
 	graphStore,
 	jobStore,
+	practiceStore,
 	settingsStore,
 };