Wave 7 close-out: the embedding ledger seam dies; notes birth via mapping lines

Part 1 — the ledger + remnants interlude (master-dispatched, owner ruling
2026-09-23: nothing configures the graph beyond chat models):

- The embedding ledger surface is gone: readEmbeddings/writeEmbeddings
  ports, the EmbeddingsLedger contract, EMBEDDINGS_FILE (config, store
  impls, graphRevision source list), and graph-deletion's vector-pruning
  step. On-disk data files stay untouched (inert history).
- The extraction embedder seam dies wholesale: ExtractionPipelineDeps
  drops the embedder dep (chat-handler + backfill/import-notes stubs go
  with it), the 'embedding' phase leaves ExtractionPhase, the usage
  capture/persist helpers die, and stages.embedding leaves
  ExtractionRunUsage (historical records keep their shape — reads are
  lenient). Extraction is two stages again, like the local-embedder era.
- Consumer-less remnants: latestQuizOutcomes + QUIZ_EVENT_KINDS,
  clusterContentId, matchClusterLabelings + Community + ClusterLabeling,
  the readClusterLabels/appendClusterLabel port seams + store impls +
  LEDGER_FILES.clusterLabels. NamingRunUsage/readNamingRuns STAY
  (standing ruling — the cost pill renders historical naming spend);
  clusterHue + MIN_CLUSTER_SIZE stay (live).
- The 'labeler' role dies: BACKGROUND_MODEL_ROLES, ROLE_SLOT, ModelRole,
  the catalog pin projections, and the UI roster strings. RolePins keys
  shrink with the union.

The notes birth fallback (owner-approved 2026-09-23, surfaced by the
interlude's live smoke: production notes imports create provenance-EMPTY
nodes (importNotes passes messages: null — the file's anchor is the
mapping line), so the provenance-only birth rule left every imported
file unclustered and invisible to the prober's session list, against the
locked file-birth ruling. A provenance-empty node now births under its
FIRST mapping line's chatId (ledger order); only notes-born nodes ever
hit the fallback (chat nodes always carry provenance; a re-encountered
node's link-append re-opens the provenance rule). Deletion-safe: mapping
lines of deleted chats are stripped alongside their provenance.

Live-verified on port 3004: a fresh notes import clusters under its
src_ id with the file path as the label, end to end with no embedder.

845/845 tests, typecheck/lint/build green.
EOF
)

src/core/session-clusters.ts

@@ -21,14 +21,24 @@
  *
  * Notes imports are birth-sessions too (owner ruling 2026-09-23): their
  * nodes carry a "src_…" source id instead of a chat id; the caller labels
- * them with the file's path (kind: file in the session views).
+ * them with the file's path (kind: file in the session views). The
+ * mapping-line fallback (owner-approved 2026-09-23, surfaced by the
+ * interlude's live smoke): production notes imports create provenance-
+ * EMPTY nodes (the notes front door passes messages: null — the file's
+ * anchor is the mapping line), so a provenance-empty node's birth session
+ * is its FIRST mapping line's chatId in ledger order. Only notes-born
+ * nodes ever hit the fallback — chat extraction always writes provenance
+ * points, and a re-encountered node's link-append adds one, which
+ * re-opens the provenance rule above. Deletion-safe: a deleted chat's
+ * mapping lines are stripped alongside its provenance; notes mappings
+ * survive because files are never deleted.
  *
  * Browser-safe pure logic (no node builtins, no fs): same contract style
  * as the rest of the core graph derivation. Not re-exported through the
  * core barrel — the shell reads clusters from the snapshot; only the
  * server-side derivations (snapshot, settle) and tests import this.
  */
-import type { GraphNode } from "./contracts.ts";
+import type { GraphNode, NodeMapping } from "./contracts.ts";
 
 /** One birth-session cluster: the session id + the SORTED member node
  *  ids born in it. The shape is deliberately the { members } contract
@@ -41,11 +51,23 @@ export interface BirthSessionCluster {
 	members: string[];
 }
 
-/** A node's birth session: its first surviving provenance point's chatId,
- *  or null when the node carries no provenance at all (unclustered). */
-export function birthSessionOf(node: GraphNode): string | null {
+/** A node's birth session: its first surviving provenance point's chatId;
+ *  for a provenance-empty node, its first mapping line's chatId (the notes
+ *  fallback — see the module doc). null when neither exists. */
+export function birthSessionOf(node: GraphNode, mappingFallback?: ReadonlyMap<string, string>): string | null {
 	const first = node.provenance[0];
-	return first === undefined ? null : first.chatId;
+	if (first !== undefined) return first.chatId;
+	return mappingFallback?.get(node.id) ?? null;
+}
+
+/** The notes fallback index: nodeId → the chatId of the node's FIRST
+ *  mapping line in ledger order. */
+export function birthMappingIndex(mappings: readonly NodeMapping[]): Map<string, string> {
+	const index = new Map<string, string>();
+	for (const mapping of mappings) {
+		if (!index.has(mapping.graphNodeId)) index.set(mapping.graphNodeId, mapping.chatId);
+	}
+	return index;
 }
 
 /**
@@ -53,15 +75,20 @@ export function birthSessionOf(node: GraphNode): string | null {
  * (first line defines — the getGraph rule), groups by birth session, and
  * returns the groups size-desc with members sorted (deterministic output:
  * ties break by session id ascending). Nodes with no surviving provenance
- * are left unclustered — they belong to no session the graph can name.
+ * fall back to their first mapping line (notes-born); a node with neither
+ * is left unclustered — it belongs to no session the graph can name.
  */
-export function birthSessionPartition(nodes: readonly GraphNode[]): BirthSessionCluster[] {
+export function birthSessionPartition(
+	nodes: readonly GraphNode[],
+	mappings: readonly NodeMapping[] = [],
+): BirthSessionCluster[] {
+	const mappingFallback = birthMappingIndex(mappings);
 	const members = new Map<string, string[]>();
 	const seen = new Set<string>();
 	for (const node of nodes) {
 		if (seen.has(node.id)) continue;
 		seen.add(node.id);
-		const birth = birthSessionOf(node);
+		const birth = birthSessionOf(node, mappingFallback);
 		if (birth === null) continue;
 		const bucket = members.get(birth);
 		if (bucket) bucket.push(node.id);