fix: improve cursor telemetry accuracy under CPU load

- Replace setInterval with drift-compensating recursive setTimeout for
  cursor sampling. Under CPU load setInterval bunches or skips callbacks,
  creating irregular gaps in telemetry data.
- Add binary search + linear interpolation to export cursor lookup
  (modernFrameRenderer) matching the playback path. Was using O(n)
  nearest-neighbor which caused visible cursor jumping in exports.
- Apply same interpolation fix to extension API getCursorAt().

src/lib/exporter/modernFrameRenderer.ts

@@ -2336,21 +2336,53 @@ export class FrameRenderer {
 			return null;
 		}
 
-		let closest = telemetry[0];
-		let minDist = Math.abs(telemetry[0].timeMs - timeMs);
-		for (let i = 1; i < telemetry.length; i++) {
-			const dist = Math.abs(telemetry[i].timeMs - timeMs);
-			if (dist < minDist) {
-				minDist = dist;
-				closest = telemetry[i];
-			}
-			if (telemetry[i].timeMs > timeMs) {
-				break;
+		// Clamp to first/last sample when out of range
+		if (timeMs <= telemetry[0].timeMs) {
+			const s = telemetry[0];
+			return mapCursorToCanvasNormalized(
+				{ cx: s.cx, cy: s.cy, interactionType: s.interactionType },
+				{
+					maskRect: this.layoutCache?.maskRect,
+					canvasWidth: this.config.width,
+					canvasHeight: this.config.height,
+				},
+			);
+		}
+		if (timeMs >= telemetry[telemetry.length - 1].timeMs) {
+			const s = telemetry[telemetry.length - 1];
+			return mapCursorToCanvasNormalized(
+				{ cx: s.cx, cy: s.cy, interactionType: s.interactionType },
+				{
+					maskRect: this.layoutCache?.maskRect,
+					canvasWidth: this.config.width,
+					canvasHeight: this.config.height,
+				},
+			);
+		}
+
+		// Binary search for surrounding samples
+		let lo = 0;
+		let hi = telemetry.length - 1;
+		while (lo < hi - 1) {
+			const mid = (lo + hi) >> 1;
+			if (telemetry[mid].timeMs <= timeMs) {
+				lo = mid;
+			} else {
+				hi = mid;
 			}
 		}
 
+		const a = telemetry[lo];
+		const b = telemetry[hi];
+		const span = b.timeMs - a.timeMs;
+
+		// Linear interpolation between samples
+		const t = span > 0 ? (timeMs - a.timeMs) / span : 0;
+		const cx = a.cx + (b.cx - a.cx) * t;
+		const cy = a.cy + (b.cy - a.cy) * t;
+
 		return mapCursorToCanvasNormalized(
-			{ cx: closest.cx, cy: closest.cy, interactionType: closest.interactionType },
+			{ cx, cy, interactionType: a.interactionType },
 			{
 				maskRect: this.layoutCache?.maskRect,
 				canvasWidth: this.config.width,

src/lib/extensions/extensionHost.ts

@@ -854,17 +854,31 @@ export class ExtensionHost {
 				const t = host._cursorTelemetry;
 				if (!t || t.length === 0) return null;
 
-				let closest = t[0];
-				let minDist = Math.abs(t[0].timeMs - timeMs);
-				for (let i = 1; i < t.length; i++) {
-					const dist = Math.abs(t[i].timeMs - timeMs);
-					if (dist < minDist) {
-						minDist = dist;
-						closest = t[i];
+				if (timeMs <= t[0].timeMs) return { ...t[0] };
+				if (timeMs >= t[t.length - 1].timeMs) return { ...t[t.length - 1] };
+
+				let lo = 0;
+				let hi = t.length - 1;
+				while (lo < hi - 1) {
+					const mid = (lo + hi) >> 1;
+					if (t[mid].timeMs <= timeMs) {
+						lo = mid;
+					} else {
+						hi = mid;
 					}
-					if (t[i].timeMs > timeMs) break;
 				}
-				return { ...closest };
+
+				const a = t[lo];
+				const b = t[hi];
+				const span = b.timeMs - a.timeMs;
+				const frac = span > 0 ? (timeMs - a.timeMs) / span : 0;
+
+				return {
+					...a,
+					cx: a.cx + (b.cx - a.cx) * frac,
+					cy: a.cy + (b.cy - a.cy) * frac,
+					timeMs,
+				};
 			},
 
 			getSmoothedCursor() {