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/session-clusters.test.ts

@@ -11,7 +11,7 @@
 
 import assert from "node:assert/strict";
 import { describe, it } from "node:test";
-import type { GraphEdge, GraphNode, NotesSource, ChatSummary } from "../src/core/contracts.ts";
+import type { GraphChain, GraphNode, NotesSource, ChatSummary } from "../src/core/contracts.ts";
 import type { ChatStore, GraphStore } from "../src/core/ports.ts";
 import {
 	graphSessionIndex,
@@ -40,10 +40,10 @@ function fakeStores(
 		sources?: Array<Pick<NotesSource, "id" | "filePath" | "importedAt">>;
 	} = {},
 ): GraphDeps {
-	const edges: GraphEdge[] = [];
+	const chains: GraphChain[] = [];
 	const graphStore: GraphStore = {
 		readNodes: () => nodes,
-		readEdges: () => edges,
+		readChains: () => chains,
 		readCheckpoints: () => [],
 		readCoverage: () => [],
 		readSources: () =>