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/render-chain-table.test.tsx

@@ -29,7 +29,7 @@ import { renderToStaticMarkup } from "react-dom/server";
 import { describe, it } from "node:test";
 import { ChainTableBody } from "../components/chain-table.tsx";
 import type { ChainSort } from "../components/chain-table.tsx";
-import type { GraphEdge, GraphNode } from "../src/core/contracts.ts";
+import type { GraphChain, GraphNode } from "../src/core/contracts.ts";
 
 /** The fixture graph — two follows-chains + one standalone:
  *  a → c1 ← b  (chain one, conclusion born chat1), d → e (chain two,
@@ -58,15 +58,14 @@ const NODES: GraphNode[] = [
 	node("orphan", "Orphan", "Statement O.", null),
 ];
 
-const EDGES: GraphEdge[] = [
-	{ from: "a", to: "c1", type: "follows" },
-	{ from: "b", to: "c1", type: "follows" },
-	{ from: "d", to: "e", type: "follows" },
-] as GraphEdge[];
+const CHAINS: GraphChain[] = [
+	{ id: "c1", premises: ["a", "b"], conclusion: "c1", provenance: [], createdAt: "2026-09-20T10:00:00.000Z" },
+	{ id: "e", premises: ["d"], conclusion: "e", provenance: [], createdAt: "2026-09-20T10:00:00.000Z" },
+];
 
 const BASE = {
 	nodes: NODES,
-	edges: EDGES,
+	chains: CHAINS,
 	sessions: { chat1: "Ethics lesson", chat2: "Second chat" },
 	fileSources: { notes1: "notes/raw/ethics.md" },
 	query: "",