fix(engine): Linux GPU path uses deprecated EGL + NVENC probe fails on data-center GPUs (#1504)

* fix(engine): use ANGLE-EGL for Linux GPU path, bump NVENC probe size

Chrome 131+ rejects --use-gl=egl in headless shell; the GPU process
exits and the renderer silently falls back to SwiftShader. Switch to
(gl=angle, angle=gl-egl) which is on the headless-shell allowlist,
and add --ignore-gpu-blocklist + --disable-software-rasterizer so
data-center GPUs (L4/T4/A10) are not blocked.

Also bump the NVENC probe frame from 16×16 to 320×240 — NVIDIA
data-center cards require ≥257 on each dimension and reject the
smaller size with "Frame Dimension less than the minimum supported
value", causing the encoder probe to silently fall back to libx264.

Closes #1493

* fix(engine): address review feedback — probe test, observability, comments

- Export getProbeArgs and add test pinning 320×240 probe dimensions
  across all 5 GPU encoder backends (nvenc/videotoolbox/vaapi/qsv/amf)
- Add driver/SKU rationale comment on the probe size constant with
  context about NVIDIA data-center card behavior vs documented minimums
- Add rationale comment on --ignore-gpu-blocklist (operator opted into
  hardware mode explicitly)
- Log resolved GL flags at browser launch for GPU fallback observability

packages/engine/src/utils/gpuEncoder.test.ts

@@ -3,6 +3,7 @@ import { afterEach, describe, expect, it, vi } from "vitest";
 import {
   getCompiledGpuEncoders,
   getGpuEncoderName,
+  getProbeArgs,
   mapPresetForGpuEncoder,
   selectUsableGpuEncoder,
 } from "./gpuEncoder.js";
@@ -127,3 +128,13 @@ describe("mapPresetForGpuEncoder", () => {
     });
   });
 });
+
+describe("getProbeArgs", () => {
+  it("uses 320x240 probe dimensions for all GPU encoders", () => {
+    const encoders = ["nvenc", "videotoolbox", "vaapi", "qsv", "amf"] as const;
+    for (const encoder of encoders) {
+      const args = getProbeArgs(encoder);
+      expect(args).toContain("color=size=320x240:rate=1:duration=1");
+    }
+  });
+});

packages/engine/src/utils/gpuEncoder.ts

@@ -108,7 +108,17 @@ export function getGpuEncoderName(encoder: GpuEncoder, codec: "h264" | "h265"):
   }
 }
 
-function getProbeArgs(encoder: ConcreteGpuEncoder): string[] {
+// Minimum probe dimensions must clear every GPU encoder's hardware minimum.
+// NVIDIA data-center SKUs (L4/T4/A10/A100) reject frames below ~257px on
+// either dimension with "Frame Dimension less than the minimum supported
+// value" (observed on driver 595.58.03, CUDA 13.2). The documented SDK
+// minimums (145×49 H.264, 129×33 HEVC) are lower, but the driver enforces
+// a stricter per-SKU alignment. 320×240 clears all known GPU encoder
+// minimums (NVENC, VideoToolbox, VAAPI, QSV, AMF) while staying cheap.
+const GPU_PROBE_WIDTH = 320;
+const GPU_PROBE_HEIGHT = 240;
+
+export function getProbeArgs(encoder: ConcreteGpuEncoder): string[] {
   const args = [
     "-hide_banner",
     "-loglevel",
@@ -116,7 +126,7 @@ function getProbeArgs(encoder: ConcreteGpuEncoder): string[] {
     "-f",
     "lavfi",
     "-i",
-    "color=size=16x16:rate=1:duration=1",
+    `color=size=${GPU_PROBE_WIDTH}x${GPU_PROBE_HEIGHT}:rate=1:duration=1`,
     "-frames:v",
     "1",
     "-an",