Make the video SynthID operating point measurable and hard to move silently

The shipped profile was certified by one oracle row, but only noise_std was
pinned: long_side and fps -- two thirds of what the verifier was actually shown
-- could move with a green suite. The test now derives the pin from
data/evaluations/video-synthid-oracle.csv, so a default without a certifying row
fails.

The certified profile is a perturbation-to-signal ratio, not a bare noise_std.
sd-vae-ft-mse publishes no scaling_factor key, so 0.18215 comes from the
AutoencoderKL class default under an upper-unbounded diffusers pin. The loader
now gates that value, carries it on VideoVaeRuntime, and passes it into encode
and decode so the validated value is the applied value. video_synthid_sweep.py
loads through the same function: the harness producing the certified rows was
the one path exempt from the gate it exists to feed.

psnr_db is measured against the already-resized frame and before the encoder, so
it cannot see the downscale, the decimation, or the codec, and no in-loop metric
can. scripts/video_fidelity_probe.py scores the delivered file end to end,
streaming the way the engine does and sharing its frame-selection rule rather
than copying it -- a frame-count check cannot catch a rule that reorders frames
without changing how many.

The manifest gains source geometry, vae, track, verbatim verdict and session
fields. The two 2026-07-31 rows keep them empty: they were never recorded and
are not recoverable. Verdicts now have four states, because the verifier's
unclear reading logged as not_detected is the silent regression the manifest
exists to prevent.

docs/video-synthid-quality-research.md records the research behind this: the
noise axis is worth about 2 dB and is nearly exhausted, resolution is the real
prize but is an uncertified destruction axis rather than a free win, and every
proposed autoencoder swap was refuted. First local measurements included.

Verified: engine output is byte-identical before and after the refactor on a
locally built clip, at noise_std 0.00 and 0.15.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.claude/rules/development.md

@@ -82,4 +82,25 @@ Before changing anything in the detection path, record the detectors' exact verd
 over a local sample first and diff them after. A refactor here is only correct if that
 record is byte-identical, and a green test suite does not establish that on its own.
 
+## A certified operating point is data, not a constant
+
+The video SynthID default is only meaningful as a row in
+`data/evaluations/video-synthid-oracle.csv`, so
+`test_shipped_defaults_match_a_certified_manifest_row` derives the pin from that
+manifest instead of restating literals. Pinning `noise_std` alone had let `long_side`
+and `fps` -- two thirds of what the oracle was actually shown -- move with a green
+suite.
+
+The certified profile is a perturbation-to-signal ratio, not a bare `noise_std`, so
+the latent scaling factor is gated in `load_video_vae_runtime`, carried on
+`VideoVaeRuntime`, and passed into encode and decode: the validated value and the
+applied value are one measurement. Anything that produces oracle evidence loads
+through that function. `video_synthid_sweep.py` hand-rolled the load and was the one
+path exempt from the gate it exists to feed, which is exactly backwards.
+
+Prove a video-path refactor the same way the detection path is proven, and without
+needing an oracle carrier: build a clip from a tracked fixture with ffmpeg, run the
+engine before and after, and require an identical output sha256. Keep the generated
+media outside the repository.
+
 Environment setup, dependency recovery, CI behavior, and fixture policy: [`../../docs/development.md`](../../docs/development.md).