chains-primary: stored chains replace typed edges; edges become a derived view

Owner rulings 2026-09-30: reasoning chains are the stored truth; the graph
canvas renders a lossy derived image. No migration, no backcompat — old
graph data was wiped.

- contracts: GraphChain (arg_-minted id, premises, conclusion) replaces
  typed edges; GraphEdge survives only as the untyped derived arrow;
  GraphSnapshot carries stored chains; supports edges retired entirely
- store: chains.jsonl ledger (edges.jsonl purged at load alongside legacy
  events.jsonl); readChains/appendChain seam; merge dedupes by
  (sorted premises, conclusion), re-assertions append under the same arg_ id
- core: deriveView mints implication nodes (name ⟹, statement via
  formatChainLine) plus premise→imp→conclusion arrows; clusters stay
  proposition-only (birthSessionPartition); chainsForSelection now groups
  STORED chains (chain rides iff every member selected) — independent
  arguments for one conclusion never union; chainsToGraph/chainsToEdges die
- extraction: extractor submits chains natively (expandFindings deleted);
  auditor receives propositions only; supports fan-out gone
- deletion: full death of a proposition kills every referencing chain;
  arg_-prefixed node deletes kill that chain; chainsRemoved stats
- UI: every arrow renders the ⟹ double shaft; supports toggle + thickness
  slider and detail-card edge sections deleted (card keeps name +
  proposition); chain table + practice pipeline read stored chains
- tests: 1188 green across the new seams

config.ts

@@ -60,11 +60,12 @@ export type ModelRole =
 
 /**
  * The LIVE graph store (src/store/graph.ts) — owned by the app's extraction
- * pipeline. Line shapes: src/core/contracts.ts.
+ * pipeline. Line shapes: src/core/contracts.ts. CHAINS ARE THE STORED
+ * TRUTH (owner, 2026-09-30): edges are a derived view, never stored.
  */
 export const LEDGER_FILES = {
 	nodes: path.join(PATHS.graph, "nodes.jsonl"),
-	edges: path.join(PATHS.graph, "edges.jsonl"),
+	chains: path.join(PATHS.graph, "chains.jsonl"),
 	sessions: path.join(PATHS.graph, "sessions.jsonl"),
 	/** Per-message extraction coverage (contracts/graph.ts CoverageEntry;
 	 *  the checkpoint's replacement for chat extractions — notes keep