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

scripts/backfill-graph.ts

@@ -27,7 +27,7 @@ async function main() {
 			const stats = await pipeline.runExtraction(chat.id);
 			console.log(
 				`[backfill] ${chat.id} (${chat.title.slice(0, 40)}): ` +
-					`${stats.nodes} nodes, ${stats.edges} edges in ${((Date.now() - t0) / 1000).toFixed(1)}s`,
+					`${stats.nodes} nodes, ${stats.chains ?? 0} chains in ${((Date.now() - t0) / 1000).toFixed(1)}s`,
 			);
 		} catch (err) {
 			console.error(`[backfill] ${chat.id} FAILED:`, err);
@@ -35,7 +35,7 @@ async function main() {
 	}
 	const graph = getGraph(graphStore);
 	console.log(
-		`[backfill] done: ${graph.nodes.length} nodes, ${graph.edges.length} edges`,
+		`[backfill] done: ${graph.nodes.length} nodes, ${graph.chains.length} chains`,
 	);
 }