Option A: incremental anchored force layout, persisted server-side

The layout now EVOLVES instead of reshuffling on every extraction. A pure
core module (src/core/layout.ts) warm-starts forceAtlas2 from the positions
ledger (data/graph/positions.json, read-modify-write like sources.json,
updatedAt stamp for future live-physics conflict handling), re-blends every
previously-positioned node toward its anchor each iteration (retention 0.85
- old placements stay recognizable, strong forces still win), seeds new
nodes near their positioned neighbors, and seeds orphans on the rim outside
the core so they read as unconnected knowledge.

- Port/store: GraphStore.readPositions/writePositions; snapshot carries
  positions when the ledger exists (optional - fresh graphs unchanged).
- Extraction pipeline settles synchronously after every merge (chat and
  notes paths), before usage persist - the next /api/graph fetch already
  reflects new nodes.
- View: the ledger IS the layout; the local one-shot FA2 pass survives
  only as a fallback when no ledger exists.
- scripts/replay-layout.ts: simulates organic growth by replaying the
  current 338-node graph one imported file at a time (real import order,
  edges activate when both endpoints exist). Real run: 338 positions in
  0.3s, 90-degree-0 orphans collecting at mean r=338 vs connected r=194.
- Also restores the cluster region fills (hull background colors) that
  6692b19 had removed on a misreading of "no glow".

Live-verified: fit zoom derives from the ledger bbox, pickAt aligns with
ledger positions at every probe, hover/click/detail-card work, 408/408
tests, typecheck/lint/build green.

Future live physics stays cheap by design: the same pure function is the
merge-time settler and the future client simulator; a pinned node is just
an anchor with full retention.

components/graph-canvas.ts

@@ -129,10 +129,16 @@ const PICKING_PAD = 10;
 /** Click vs drag threshold: pointer travel under this many px is a click. */
 const CLICK_SLOP = 4;
 
-/** Cluster hull styling: a SOLID viewer-relative border only. The layered
- *  low-alpha fills (the "glow") were removed on user request — they read
- *  as jagged up close (straight-line hull approximation) and are better
- *  revisited once the layout settles. */
+/** Cluster hull styling: layered low-alpha fills tint each region, plus
+ *  a SOLID viewer-relative border. (The fills were briefly removed on a
+ *  misreading of "no glow": they are the REGION BACKGROUNDS, not the
+ *  glow. The jagged-when-zoomed look is the straight-line hull
+ *  approximation, polishable later once the layout settles.) */
+const HULL_FILL_LAYERS = [
+	[24, 0.025],
+	[19, 0.04],
+	[14, 0.06],
+] as const;
 const HULL_STROKE_PAD = 14;
 const HULL_CORNER_RADIUS = 9;
 const REGION_LABEL_SIZE = 14;
@@ -356,6 +362,7 @@ export class GraphCanvasRenderer {
 	private shaftsPath: Path2D = new Path2D();
 	private headsPath: Path2D = new Path2D();
 	private hullShapes: {
+		fill: Path2D[];
 		stroke: Path2D;
 		label: string | null;
 		cx: number; // world centroid
@@ -704,7 +711,8 @@ export class GraphCanvasRenderer {
 		this.headsPath = heads;
 
 		// Cluster hulls: convex hull of member positions (world = raw data
-		// coordinates — no projection at all) and a SOLID boundary path.
+		// coordinates, no projection at all), three offset fill layers,
+		// and a SOLID boundary path.
 		this.hullShapes = [];
 		for (const cluster of data.clusters) {
 			const pts: Pt[] = [];
@@ -715,11 +723,16 @@ export class GraphCanvasRenderer {
 			}
 			const hull = convexHull(pts);
 			if (hull.length < 3) continue;
+			const fills = HULL_FILL_LAYERS.map(([pad]) => {
+				const path = new Path2D();
+				traceRoundedPath(path, offsetPolygon(hull, pad * s), HULL_CORNER_RADIUS * s);
+				return path;
+			});
 			const stroke = new Path2D();
 			traceRoundedPath(stroke, offsetPolygon(hull, HULL_STROKE_PAD * s), HULL_CORNER_RADIUS * s);
 			const cx = pts.reduce((sum, p) => sum + p.x, 0) / pts.length;
 			const cy = pts.reduce((sum, p) => sum + p.y, 0) / pts.length;
-			this.hullShapes.push({ stroke, label: cluster.label || null, cx, cy, hue: clusterHue(cluster.id) });
+			this.hullShapes.push({ fill: fills, stroke, label: cluster.label || null, cx, cy, hue: clusterHue(cluster.id) });
 		}
 
 		// Label wrapping: measureText once per node per geometry build,
@@ -764,9 +777,13 @@ export class GraphCanvasRenderer {
 		const regionAlpha = 1 - nodeAlpha;
 		const { nodeR, glowOuter } = this.world;
 
-		// 1. Cluster hulls: cached world paths — SOLID viewer-relative
-		// hairline border only (no fill glow, removed for now).
+		// 1-2. Cluster hulls: cached world paths — soft layered region
+		// fills, then a SOLID viewer-relative hairline border.
 		for (const hs of this.hullShapes) {
+			for (let i = 0; i < hs.fill.length; i++) {
+				ctx.fillStyle = `hsla(${hs.hue}, 65%, 60%, ${HULL_FILL_LAYERS[i]![1]})`;
+				ctx.fill(hs.fill[i]!);
+			}
 			ctx.strokeStyle = `hsla(${hs.hue}, 65%, 62%, 0.5)`;
 			ctx.lineWidth = 1.5 / k; // constant screen px
 			ctx.stroke(hs.stroke);