feat(engine): open drawElement fast capture to Windows hardware GPU

Widen the default-on drawElement clamp from darwin-only to darwin|win32
(still requiring a non-software-GPU browser). The darwin restriction was a
validation envelope, not an architectural limit — the CanvasDrawElement
Chrome flag ships on every platform, and every safety layer that made the
macOS default-on release (v0.7.38) survivable is platform-neutral:
compile-time gates, the SwiftShader init gate, per-render worker-encode
self-verification with screenshot fallback, and the blank guard. Worst case
on an unvalidated D3D11 backend is the same as on Metal: verify catches a
bad frame and the render re-runs on the screenshot baseline.

Why now: 30-day telemetry shows ~206k non-CI hardware-GPU Windows renders
(~78% of the win32 fleet, 18k installs) held on the slow screenshot path by
the clamp — the second-largest perf population after macOS, carrying ~1,550
capture-hours/month in the DE-eligible >=700-frame band alone at a measured
~2x speedup opportunity.

Instrumentation for the new cohort: drawElement session init now records the
raw WebGL UNMASKED_RENDERER_WEBGL string (detectSwiftShader generalized to
detectGpuBackend — same single evaluate, the string was previously read and
discarded) and threads it session -> CapturePerfSummary -> RenderPerfSummary
-> render_complete as `gpu_renderer`. drawElement damage proved
compositor-backend-specific throughout the macOS rollout, so D3D11-cohort
failures must cluster by ANGLE backend + GPU vendor (NVIDIA/AMD/Intel), not
just `os`.

The two DE clamp branches are extracted into a pure, unit-tested
`resolveDefaultDrawElement` (platform + GPU mode + worker-encode + explicit
opt-in), which also drops resolveConfig's cyclomatic complexity. The win32
streaming-encode compound tests collapse onto one shared helper.

Linux stays excluded: that fleet is headless/Docker SwiftShader, where DE
has no speedup and known rendering defects. Kill switches unchanged:
PRODUCER_EXPERIMENTAL_FAST_CAPTURE=false, --experimental-fast-capture=false.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

packages/engine/src/services/frameCapture.ts

@@ -34,7 +34,7 @@ import {
   shouldDefaultCaptureBeyondViewport,
 } from "./screenshotService.js";
 import {
-  detectSwiftShader,
+  detectGpuBackend,
   injectDrawElementCanvas,
   captureDrawElementFrame,
   resolveDrawElementCaptureMode,
@@ -146,6 +146,13 @@ export interface CaptureSession {
   config?: Partial<EngineConfig>;
   /** True if running on SwiftShader (detected at init). Undefined before init. */
   isSwiftShader?: boolean;
+  /**
+   * Raw WebGL UNMASKED_RENDERER_WEBGL string, captured alongside the
+   * SwiftShader probe at DE session init (e.g. "ANGLE (NVIDIA, D3D11 ...)").
+   * Surfaces in CapturePerfSummary → render telemetry so backend-specific
+   * drawElement damage (Metal vs D3D11 vs GL) clusters attributably.
+   */
+  gpuRenderer?: string;
   /** drawElementImage canvas was injected and is ready for capture. */
   drawElementReady?: boolean;
   /**
@@ -704,7 +711,9 @@ async function initDrawElementOrTransparentBackground(
     );
   }
   if (useDrawElement) {
-    session.isSwiftShader = await detectSwiftShader(page);
+    const gpuBackend = await detectGpuBackend(page);
+    session.isSwiftShader = gpuBackend.isSwiftShader;
+    session.gpuRenderer = gpuBackend.renderer ?? undefined;
     const transparent = session.options.format === "png";
     async function routeToFallback(): Promise<void> {
       session.captureMode = session.launchCaptureMode;
@@ -3784,6 +3793,7 @@ export function getCapturePerfSummary(session: CaptureSession): CapturePerfSumma
     beginFrameNoDamage: session.beginFrameNoDamageCount,
     beginFrameHasDamage: session.beginFrameHasDamageCount,
     captureMode: session.captureMode,
+    gpuRenderer: session.gpuRenderer,
     deGateReason: session.deGateReason,
     deFallbackTrigger: session.deFallbackTrigger,
     deWorkerEncode: session.workerEncodeEnabled ?? false,