docs(long-form): gate the R3 route on the PRINFRA-694 fixture, record the hardening

- tests/long-form/README: the two-clip in-flow fixture, the per-frame luma
  check over clip B, the 2026-09-18 matrix (branch -w 1 / -w 2 disk / -w 2
  streaming / absolute CSS / published 0.8.44 all 0 of 100 black; 0.8.35 and
  0.8.36 99 of 100 as the positive control; fixed in 0.8.37 by #3893), and
  why the multi-worker trace's captureMode was not the engine's mode.
- HANDOFF: R3 row now reads "hardening built"; new section 4a lists the
  guards, the 694 gate result and what the flip still needs; counts refreshed.
- cli.mdx: the parallel-stream paragraph says the opt-in path now retries a
  stalled pool or a mid-stream encoder exit on a fresh single-worker session.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/packages/cli.mdx

@@ -938,9 +938,10 @@ is now per-frame rather than whole-render, so it no longer applies.
 Multi-worker mp4/mov renders also stream where capture runs BeginFrame — Linux
 on the bundled `chrome-headless-shell`, at DPR 1, with no forced screenshot. On
 macOS and Windows, multi-worker capture is screenshot-based and still writes
-frames to disk unless you opt in with `HF_CAPTURE_PARALLEL_STREAM=true`; that
-path works but currently fails more often than the disk path, so it is not yet
-the default.
+frames to disk unless you opt in with `HF_CAPTURE_PARALLEL_STREAM=true`. That
+path now retries a stalled worker pool or a mid-stream encoder exit on a fresh
+single-worker session instead of failing the render, but it stays opt-in until
+its error rate matches the disk path's.
 
 Renders still land on the disk path when the output is `png-sequence` or `gif`,
 when HDR or shader-transition layering is in play, and when streaming is