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

tests/usage.test.ts

@@ -244,7 +244,7 @@ test("appendExtractionUsage: appends, idempotent per startedAt, not in list payl
 		startedAt,
 		at: "2026-09-09T00:00:00Z",
 		durationMs: 42_000,
-		stats: { nodes: 1, edges: 2 },
+		stats: { nodes: 1, chains: 2 },
 		stages: {
 			extractor: { model: "openrouter:z-ai/glm-5.3-flash", steps: [call({ inputTokens: 100, outputTokens: 10 })] },
 			auditor: { model: "openrouter:z-ai/glm-5.3-flash", steps: [call({ inputTokens: 50, outputTokens: 5 })] },
@@ -281,7 +281,7 @@ test("appendSourceUsage: appends to the source record, idempotent per startedAt,
 			startedAt,
 			at: "2026-09-13T00:00:00Z",
 			durationMs: 42_000,
-			stats: { nodes: 1, edges: 2 },
+			stats: { nodes: 1, chains: 2 },
 			stages: {
 				extractor: { model: "openrouter:z-ai/glm-5.3-flash", steps: [call({ inputTokens: 100, outputTokens: 10 })] },
 				auditor: { model: "openrouter:z-ai/glm-5.3-flash", steps: [call({ inputTokens: 50, outputTokens: 5 })] },