practice slice 1 — contracts, the chain bijection, coverage

src/core/contracts/practice.ts (new domain): ReasoningChain (group by
conclusion, id = conclusion node id), PracticeParams (three 0..1 dials),
ProblemSet/Problem lifecycle, feedback-only ProblemGrade. src/core/
practice.ts: chainsForSelection (follows-only, both-ends-in-selection,
group-by-conclusion, singletons for edge-less nodes), chainsToGraph
(the exact inverse — bijection pinned), chainCoverage (the completeness
law: union of problems' chains = the set's chains; duplicated allowed).
13-test suite: diamond, premise-only, singleton, supports-invisible,
outside-endpoint, dedupe, determinism, round-trips, coverage cases.

src/core/contracts.ts

@@ -4,9 +4,10 @@
  * Split by domain (src/core/contracts/): plan.ts (the session plan tree),
  * graph.ts (the global knowledge graph + its extraction pipeline), chat.ts
  * (session records + UI extension points), jobs.ts (the async sub-agent
- * protocol). Each domain file is owned outright by its stream; schema
- * changes to a domain you don't own go through master. This barrel keeps
- * every existing import path ("@/src/core" and "./contracts.ts") unchanged.
+ * protocol), practice.ts (the practice-problem system). Each domain file is
+ * owned outright by its stream; schema changes to a domain you don't own go
+ * through master. This barrel keeps every existing import path
+ * ("@/src/core" and "./contracts.ts") unchanged.
  */
 
 export * from "./contracts/plan.ts";
@@ -15,3 +16,4 @@ export * from "./contracts/chat.ts";
 export * from "./contracts/jobs.ts";
 export * from "./contracts/probe.ts";
 export * from "./contracts/models.ts";
+export * from "./contracts/practice.ts";