feat(engine): browserGpuMode "auto" — probe WebGL once, fall back to software

When the host doesn't have a usable GPU (CI containers, eval rigs without
GPU passthrough, dev VMs), Chrome's hardware-mode WebGL flags
(`--use-gl=egl/metal/d3d11`) silently leave WebGL unavailable —
`getContext("webgl")` returns null, three.js' WebGLRenderer dies, the
canvas stays black. Surfaced today by Abhay's c2v-eval failing on a
docker render of an hf bundle that uses three.js + a custom fragment
shader.

The fix that's been there: `--use-gl=angle --use-angle=swiftshader` (CPU
software WebGL, ~5-50× slower but pixel-identical). The engine already
exposed `browserGpuMode: "software"` for this. The gap was discovery —
users had to know to pass `--no-browser-gpu` on no-GPU hosts.

This change adds `browserGpuMode: "auto"` (now the CLI default for local
renders): on first launch in the process, probe Chrome with hardware
args, check `canvas.getContext("webgl") !== null`, cache the result.
~1-2 s on first render, free on every subsequent render in the same
worker. Hardware GPUs keep their fast path; no-GPU hosts get SwiftShader
without ceremony.

Behaviour matrix:
- No flag, no env, local       → "auto" (NEW default)
- `--browser-gpu`              → "hardware" (force; errors if no GPU)
- `--no-browser-gpu`           → "software" (force SwiftShader)
- `PRODUCER_BROWSER_GPU_MODE`  → "hardware" / "software" / "auto" / unset
- Docker mode                  → forced "software" (unchanged)

Engine-config default stays "software" (conservative for embedders); the
"auto" default lives in the CLI's `resolveBrowserGpuForCli` so producer
embedders aren't surprised by a probe-on-launch.

Also adds `--enable-unsafe-swiftshader` to the software flag set —
Chrome 120+ deprecated implicit SwiftShader fallback and emits a
deprecation warning unless the flag is set explicitly. Despite the
"unsafe" name this is exactly the pre-deprecation behaviour; the rename
is about Chrome's threat model on the open web, not about the rendering
itself.

Verification:
- Engine 535/535 + CLI 256/256 (incl. new probe tests + tri-state CLI test)
- Empirical: probe on this no-GPU devbox returns "software" in 240 ms,
  cached 0 ms on subsequent calls
- Format / lint / typecheck clean across all packages

Refs the Abhay/Slack thread on c2v-eval rendering without a GPU node.

packages/engine/src/services/frameCapture.ts

@@ -18,6 +18,7 @@ import {
   releaseBrowser,
   forceReleaseBrowser,
   buildChromeArgs,
+  resolveBrowserGpuMode,
   resolveHeadlessShellPath,
   type CaptureMode,
 } from "./browserManager.js";
@@ -115,9 +116,14 @@ export async function createCaptureSession(
   const forceScreenshot = config?.forceScreenshot ?? DEFAULT_CONFIG.forceScreenshot;
   const preMode: CaptureMode =
     headlessShell && isLinux && !forceScreenshot ? "beginframe" : "screenshot";
+  const requestedGpuMode = config?.browserGpuMode ?? DEFAULT_CONFIG.browserGpuMode;
+  const resolvedGpuMode = await resolveBrowserGpuMode(requestedGpuMode, {
+    chromePath: headlessShell ?? undefined,
+    browserTimeout: config?.browserTimeout,
+  });
   const chromeArgs = buildChromeArgs(
     { width: options.width, height: options.height, captureMode: preMode },
-    config,
+    { ...config, browserGpuMode: resolvedGpuMode },
   );
 
   const { browser, captureMode } = await acquireBrowser(chromeArgs, config);