fix(producer): segmented and sequential capture check drawElement samples - #7255

What

Segmented capture, and the streaming stage's sequential loop (drawElement without worker encode), now run the drawElement self-check: at each sampled frame, the drawElement frame is compared against the screenshot taken before the canvas was injected, and a frame below the PSNR floor fails the render with DrawElementVerificationError before it reaches the encoder.

Why

The check existed only inside the streaming stage's drain guard. The segmented stage and the streaming sequential loop both capture with captureFrameToBuffer and never read session.deVerifyFrames, so a drawElement render on either path had no quality net: a damaged frame was encoded and shipped. This has to land before segmented capture becomes the default.

Related work

Refs the segmented capture work (Phase 2). Does not touch the GPU-less Linux BeginFrame hang or its stack.

How

• The sample comparison moves out of createDrainFrameGuard into one exported function, verifyDrawElementSample. The drain guard calls it exactly as before, so the worker-encode drain does not change.
• captureSegmentFrames and the streaming sequential loop call the same function on every frame before writing it. Sessions not in drawElement mode, and frames that are not samples, return immediately.
• Only the sample check is shared. The drain guard's blank-frame part stays streaming-only, because captureFrameToBuffer already runs the engine's own blank guard on this path.
• The orchestrator already re-renders a segmented attempt by screenshot on DrawElementVerificationError (renderOrchestrator.segmentedDeRetry.test.ts), so nothing changes there.
• Each worker session arms its own ground truth at the same sample indices, recycled sessions included. Whichever worker captures a sampled frame checks it.

Test plan

• Segmented unit tests added. They run 2 workers over 9 frames with samples armed at frames 2 and 7.
  • One test checks that both samples are compared against their own ground truth.
  • The other checks that a sample below the floor fails the render, wrapping DrawElementVerificationError, and is never written to the encoder.
• Unit tests added for the streaming sequential loop: the armed sample at frame 3 is compared, and a sample below the floor fails the render with DrawElementVerificationError.
• fails without the fix: "drawElement self-verify > compares every armed sample against its ground truth" and "drawElement self-verify > fails the render before a sample below the floor is encoded" (captureSegmentedStage.test.ts); "drawElement self-verify on the sequential loop > compares the armed sample against its ground truth" and "drawElement self-verify on the sequential loop > fails the render on a sample below the floor" (captureStreamingStage.test.ts). The segmented pair also goes red when the shared check returns without comparing. All pass again once restored.
• captureSegmentedStage.test.ts and captureStreamingStage.test.ts: 73 pass, 3 runs in a row.
• renderOrchestrator.segmentedDeRetry.test.ts (vitest): 7 pass, 3 runs in a row.
• tsc --noEmit (producer), oxlint and oxfmt on the changed files: clean. Run on a Linux box.
• Manual render: not done. The change routes frames through a check that is already exercised on the streaming path.
• Comments follow CONTRIBUTING.md.