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

src/server/graph.ts

@@ -75,7 +75,7 @@ export function graphRevision(): number {
 	let revision = 0;
 	for (const file of [
 		LEDGER_FILES.nodes,
-		LEDGER_FILES.edges,
+		LEDGER_FILES.chains,
 		LEDGER_FILES.sessions,
 		LEDGER_FILES.coverage,
 		POSITIONS_FILE,