fix(data-viz): SSR-safe gradient id + NaN-safe ProgressRing

Two bugs in @bytelyst/data-viz@0.1.0 surfaced during a cross-repo audit:

1. Sparkline used `useMemo(() => \`bl-spark-${Math.random()...}\`)`
   for its <linearGradient> id. Math.random() produces different values
   during Next.js SSR vs client hydration, triggering React hydration
   mismatches (and broken gradient refs on first paint). Swap for
   React's `useId()` — deterministic across server + client.

   Also: with a single-element series, `data.length > 0` was true but
   the early-return branch left `lastX=lastY=0`, painting a stale dot
   at the SVG origin. Tighten the guard to `data.length >= 2`.

2. ProgressRing's `Math.max(0, Math.min(1, value))` propagated NaN
   when callers passed `NaN` or `Infinity` (e.g. division-by-zero
   metrics) — producing "NaN percent" in the aria-label. Guard with
   `Number.isFinite` first.

Regression tests cover all three cases — 17/17 passing.

Tests:  pnpm -F @bytelyst/data-viz test  →  17 passed

packages/data-viz/src/ProgressRing.tsx

@@ -37,7 +37,8 @@ export function ProgressRing({
   className,
   style,
 }: ProgressRingProps) {
-  const clamped = Math.max(0, Math.min(1, value));
+  const safeValue = Number.isFinite(value) ? value : 0;
+  const clamped = Math.max(0, Math.min(1, safeValue));
   const { radius, circumference, dashOffset } = useMemo(() => {
     const r = (size - thickness) / 2;
     const c = 2 * Math.PI * r;

packages/data-viz/src/Sparkline.tsx

@@ -1,4 +1,4 @@
-import { useMemo, type CSSProperties } from 'react';
+import { useId, useMemo, type CSSProperties } from 'react';
 
 export interface SparklineProps {
   /** Series values. Length 2+. */
@@ -54,9 +54,7 @@ export function Sparkline({
       const y = pad + innerH - ((v - min) / range) * innerH;
       return [x, y] as const;
     });
-    const path = pts
-      .map(([x, y], i) => (i === 0 ? `M${x},${y}` : `L${x},${y}`))
-      .join(' ');
+    const path = pts.map(([x, y], i) => (i === 0 ? `M${x},${y}` : `L${x},${y}`)).join(' ');
     const areaPath =
       pts.length > 1
         ? `${path} L${pts[pts.length - 1]![0]},${height} L${pts[0]![0]},${height} Z`
@@ -65,10 +63,9 @@ export function Sparkline({
     return { path, areaPath, lastX: lx, lastY: ly };
   }, [data, width, height, strokeWidth]);
 
-  const gradId = useMemo(
-    () => `bl-spark-${Math.random().toString(36).slice(2, 8)}`,
-    [],
-  );
+  // useId() is SSR-stable; using Math.random() here would mismatch on hydration.
+  const reactId = useId();
+  const gradId = `bl-spark-${reactId.replace(/[^a-zA-Z0-9_-]/g, '')}`;
 
   return (
     <svg
@@ -100,7 +97,7 @@ export function Sparkline({
         strokeLinecap="round"
         strokeLinejoin="round"
       />
-      {showLastPoint && data.length > 0 && (
+      {showLastPoint && data.length >= 2 && (
         <circle
           cx={lastX}
           cy={lastY}

packages/data-viz/src/__tests__/data-viz.test.tsx

@@ -23,6 +23,25 @@ describe('Sparkline', () => {
     const path = screen.getByTestId('bl-sparkline').querySelector('path');
     expect(path?.getAttribute('d')).toBe('');
   });
+  it('derives gradient id from React useId, not Math.random (SSR-safe)', () => {
+    // Regression: previously used Math.random() which caused Next.js
+    // hydration mismatches. React's useId emits ids shaped like ":r3:" /
+    // "_r_3_" — deterministic per render-position. We assert the id
+    // matches that shape (only the React-id chars), with no 6-hex random
+    // suffix.
+    const { container } = render(<Sparkline data={[1, 2, 3]} />);
+    const id = container.querySelector('linearGradient')?.getAttribute('id') ?? '';
+    expect(id).toMatch(/^bl-spark-[a-zA-Z0-9_-]+$/);
+    // Math.random().toString(36).slice(2, 8) always produced exactly 6
+    // chars after `bl-spark-`. useId produces something different.
+    const suffix = id.replace(/^bl-spark-/, '');
+    expect(/^[0-9a-f]{6}$/.test(suffix)).toBe(false);
+  });
+  it('omits the last-point circle when given a single datum', () => {
+    render(<Sparkline data={[5]} />);
+    // Single-point series has no meaningful trend, so no stale (0,0) circle.
+    expect(screen.getByTestId('bl-sparkline').querySelector('circle')).toBeNull();
+  });
 });
 
 describe('BarSparkline', () => {
@@ -61,15 +80,11 @@ describe('KpiCard', () => {
   it('goodWhen=lower inverts the success/danger interpretation', () => {
     // For latency: a negative delta (faster) is good.
     const { rerender } = render(
-      <KpiCard label="latency" value="120ms" deltaPercent={-10} goodWhen="lower" />,
+      <KpiCard label="latency" value="120ms" deltaPercent={-10} goodWhen="lower" />
     );
-    expect(
-      screen.getByTestId('bl-kpi-delta').getAttribute('data-good'),
-    ).toBe('true');
+    expect(screen.getByTestId('bl-kpi-delta').getAttribute('data-good')).toBe('true');
     rerender(<KpiCard label="latency" value="120ms" deltaPercent={5} goodWhen="lower" />);
-    expect(
-      screen.getByTestId('bl-kpi-delta').getAttribute('data-good'),
-    ).toBe('false');
+    expect(screen.getByTestId('bl-kpi-delta').getAttribute('data-good')).toBe('false');
   });
   it('renders sparkline when trend provided', () => {
     render(<KpiCard label="X" value={1} trend={[1, 2, 3, 4]} />);
@@ -85,11 +100,18 @@ describe('ProgressRing', () => {
     rerender(<ProgressRing value={-1} />);
     expect(screen.getByTestId('bl-progress-ring').getAttribute('aria-label')).toBe('0 percent');
   });
+  it('treats NaN / non-finite value as 0 percent (no `NaN percent` aria-label)', () => {
+    render(<ProgressRing value={Number.NaN} />);
+    expect(screen.getByTestId('bl-progress-ring').getAttribute('aria-label')).toBe('0 percent');
+    cleanup();
+    render(<ProgressRing value={Number.POSITIVE_INFINITY} />);
+    expect(screen.getByTestId('bl-progress-ring').getAttribute('aria-label')).toBe('0 percent');
+  });
   it('renders inner content slot', () => {
     render(
       <ProgressRing value={0.5}>
         <span data-testid="center">50%</span>
-      </ProgressRing>,
+      </ProgressRing>
     );
     expect(screen.getByTestId('center').textContent).toBe('50%');
   });
@@ -103,7 +125,14 @@ describe('Heatmap', () => {
     expect(screen.getAllByTestId(/bl-heatmap-cell-/).length).toBe(14);
   });
   it('zero-value cells render with empty intensity', () => {
-    render(<Heatmap cells={[{ date: 'a', value: 0 }, { date: 'b', value: 5 }]} />);
+    render(
+      <Heatmap
+        cells={[
+          { date: 'a', value: 0 },
+          { date: 'b', value: 5 },
+        ]}
+      />
+    );
     const a = screen.getByTestId('bl-heatmap-cell-0');
     const b = screen.getByTestId('bl-heatmap-cell-1');
     expect(a.getAttribute('data-intensity')).toBe('0.00');