Detail card: per-session understanding notes next to the session list

The extractor's per-proposition understanding observations (named-word
scale: solid/shaky/accepted/...) were ledger data since the redesign,
with no surface. This exposes them at read time and shows each
session's note as a small chip on its session row in the node detail
card — the per-session confidence history the user asked for, one
observation per (node, chat), latest wins within a chat.

- GraphSnapshot.nodeObservations: nodeId -> chatId -> note, built in
  graphSnapshot() from observation events (pure read-time join).
- Sessions list rows: link + observation chip.
- Session-relative judgments stay session-scoped: they render next to
  their session, never as node state (the state model stays pending).

src/core/contracts/graph.ts

@@ -229,6 +229,11 @@ export interface GraphSnapshot {
 	sessions: Record<string, string>;
 	/** nodeId → chatIds that touched the node (click-through provenance). */
 	nodeSessions: Record<string, string[]>;
+	/** nodeId → chatId → that session's understanding observation (the
+	 *  extractor's per-session named-word note; session-relative data,
+	 *  shown next to the session list — the confidence history). Multiple
+	 *  notes for one (node, chat) collapse to the latest. */
+	nodeObservations: Record<string, Record<string, string>>;
 }
 
 /** Derive a short chat title from the learner's first message (pure).