Graph nodes gain a name; nodes are objective facts (no learner/session references)

Schema: GraphNode + ExtractedProposition carry 'name' — a short 2-6 word
header, the rendering handle. Node theory made explicit: a node is an
objective world-facing fact; the learner's relationship to it lives
entirely in the event ledger (state labels are derived, never stored).
Extractor + auditor instructions enforce it — misconceptions state the
wrong idea itself as a claim, never 'the learner believes Y'. Legacy
ledger lines without a name derive one from the statement on read.
graph-view: labels use the name; the detail panel shows name as header
and statement as body.

tests/extraction.test.ts

@@ -135,6 +135,7 @@ const FINDINGS: SessionFindings = {
 	propositions: [
 		{
 			id: "p1",
+			name: "Chain rule",
 			statement: "Chain rule intuition",
 			aliases: [],
 			kind: "truth",
@@ -143,6 +144,7 @@ const FINDINGS: SessionFindings = {
 		},
 		{
 			id: "p2",
+			name: "Applying chain rule",
 			statement: "Applying the chain rule",
 			aliases: [],
 			kind: "truth",