Graph view refinements: server-persisted settings, label size, eased zoom, resize-blank fix

Five refinements to the hand-rolled renderer, all user-directed:

1. Settings are SERVER-persisted (data/render-settings.json via new
   GET/PUT /api/graph/render-settings) - one source of truth for every
   browser, replacing localStorage. The client loads once after mount,
   saves debounced on user changes, and never lets a slow GET clobber a
   faster user (or mounting PUT defaults over the server copy). The
   shape/defaults/sanitizer moved to src/core/render-settings.ts - pure,
   shared by client, server store, and tests; every disk/network value
   passes through sanitizeRenderSettings.

2. New "Label size" control (default 12px - the previous hardcoded size).
   Font, wrap width, and line height all scale; verified live: +76k px
   with the label gate open.

3. "Labels until zoom" slider reworked: log-scaled over 0.0625-4 so the
   useful band (0.25-0.5 camera-ratio units) sits at 33%/50% of the
   track instead of crammed at the far left of the old linear 0.25-4
   slider. Display units are normalized: the default 0.5 reads as "1x",
   0.25 as "0.5x". The underlying labelZoom semantics are unchanged,
   so existing values survive.

4. Smooth wheel zoom: d3-zoom instant per-notch jumps (x1.2, bang) are
   replaced by an eased glide - wheel events accumulate a cursor-anchored
   target scale and each tick eases the camera toward it (exponential,
   120ms constant, frame-rate independent). Pan/pinch stay d3-owned; a
   pan cancels a glide in progress. Verified live: 8-frame glide.

5. Resize blanking fixed (graph vanished for the duration of a window
   drag, reappearing ~300ms after). Cause: every resize event
   reallocated the canvas backing store - which CLEARS it - and the
   redraw was deferred to rAF, which live resizes starve via relayout,
   so the canvas sat wiped for the whole drag. Fix: the ResizeObserver
   callback now redraws SYNCHRONOUSLY after reallocating (verified via
   a probe RO firing in the same delivery batch: canvas fully painted).
   kFit (the label gate 1x reference) is also recomputed on resize from
   the now-cached world bbox - the camera itself still persists.

tests/render-settings.test.ts: 10 tests pinning the sanitizer (garbage/
partial/clamps, legacy no-labelSize records) and the slider mapping
(default dead-center at 500, monotonic, exact round-trips, display
units). 402/402 green; typecheck, lint, build pass.

app/api/graph/render-settings/route.ts

@@ -0,0 +1,31 @@
+/**
+ * GET/PUT /api/graph/render-settings — server persistence for the graph
+ * view's rendering controls (node/edge/arrow/label sizes, the label-zoom
+ * threshold, the cluster-labels toggle). The file lives at
+ * data/render-settings.json (config.RENDER_SETTINGS_FILE).
+ *
+ *   GET → 200 {settings} once saved, 200 null on first boot (client keeps
+ *         in-memory defaults until the user changes something).
+ *   PUT → 200 {settings} (the sanitized copy actually persisted — the
+ *         client may echo it to stay honest with clamped values).
+ *
+ * Every body is passed through sanitizeRenderSettings (src/core) before
+ * touching disk: garbage clamps or falls back to defaults, never throws.
+ */
+import { readRenderSettings, writeRenderSettings } from "@/src/store/render-settings.ts";
+
+export const dynamic = "force-dynamic";
+
+export async function GET() {
+	return Response.json(readRenderSettings());
+}
+
+export async function PUT(request: Request) {
+	let body: unknown;
+	try {
+		body = await request.json();
+	} catch {
+		return Response.json({ error: "invalid JSON body" }, { status: 400 });
+	}
+	return Response.json(writeRenderSettings(body));
+}