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.

src/store/render-settings.ts

@@ -0,0 +1,40 @@
+/**
+ * Server-side persistence for the graph render settings (the settings-panel
+ * controls): data/render-settings.json — read-modify-write, NOT a ledger.
+ *
+ * The shape, defaults, and sanitizer live in src/core/render-settings.ts
+ * (pure, shared with the client); this module only moves bytes. Whatever
+ * comes off disk is sanitized by the caller on load — and we sanitize on
+ * write too, so the file can never hold an out-of-range value that would
+ * silently shift a slider.
+ */
+
+import fs from "node:fs";
+import path from "node:path";
+import { RENDER_SETTINGS_FILE } from "../../config.ts";
+import { DEFAULT_RENDER_SETTINGS, sanitizeRenderSettings, type GraphRenderSettings } from "../core/render-settings.ts";
+
+/** The persisted settings, or null when no file exists yet (first boot). */
+export function readRenderSettings(): GraphRenderSettings | null {
+	let raw: string;
+	try {
+		raw = fs.readFileSync(RENDER_SETTINGS_FILE, "utf-8");
+	} catch {
+		return null; // never booted / unreadable → caller uses defaults
+	}
+	try {
+		return sanitizeRenderSettings(JSON.parse(raw));
+	} catch {
+		return null; // corrupt → defaults, never crash the view
+	}
+}
+
+/** Persist settings (sanitized). Returns what was written. */
+export function writeRenderSettings(raw: unknown): GraphRenderSettings {
+	const settings = sanitizeRenderSettings(raw);
+	fs.mkdirSync(path.dirname(RENDER_SETTINGS_FILE), { recursive: true });
+	fs.writeFileSync(RENDER_SETTINGS_FILE, JSON.stringify(settings, null, "\t"), "utf-8");
+	return settings;
+}
+
+export { DEFAULT_RENDER_SETTINGS };