fix: chain table reads the stored propositions, not the derived canvas image

The owner's live test surfaced it: after importing notes, the chains view
listed implication nodes as bogus singleton "chains" — and each carried
the same arg_ id as the real stored chain, tripping React's duplicate-key
error (chain-table.tsx:372).

Root cause: graph-view passed snapshot.nodes — the derived canvas view
(propositions + implication nodes) — into ChainTableOverlay, whose
chainsForSelection turns every unchained node into a singleton chain.
An implication node is never a chain member (members are proposition
ids), so every imp node minted a bogus singleton with a colliding id;
its row was even selectable, leaking arg_ ids into practice input.

Fix (owner-approved shape): GraphSnapshot gains `propositions` — the
stored-truth node list riding beside `chains` for exactly this class of
consumer. The chain table now receives snapshot.propositions; the canvas
keeps snapshot.nodes (imp nodes are meant to be drawn). Practice pipeline
unaffected (it already read getGraph, which is propositions-only).

Test: the snapshot split pinned as an invariant in graph.test.ts — image
= truth + one imp node per fully-live stored chain; truth never carries
an arg_ id.

src/core/contracts/graph.ts

@@ -307,6 +307,10 @@ export interface GraphSnapshot {
 	/** Proposition nodes + the DERIVED implication nodes (one per chain,
 	 *  id = chain id) — what the canvas renders. */
 	nodes: GraphNode[];
+	/** The STORED proposition nodes alone (no derived implication nodes)
+	 *  — the stored truth, like `chains`: consumers that read the world
+	 *  (the chain table) take this list, never the canvas image. */
+	propositions: GraphNode[];
 	/** DERIVED untyped arrows: premise → imp → conclusion. A lossy image
 	 *  of the stored chains — rebuilt after every change, never stored. */
 	edges: GraphEdge[];