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>

data/evaluations/video-synthid-oracle.csv

@@ -1,3 +1,3 @@
-date,source_url,source_sha256,duration_seconds,source_verdict,noise_std,long_side,fps,seed,output_sha256,output_verdict,psnr_db,temporal_residual_ratio
-2026-07-31,https://storage.googleapis.com/gdm-deepmind-com-prod-public/media/media/veo__veo-3__off-road.mp4,79a552b9406a079682440c31f14d33a10ba8e1b8b2e96425f5de70f63350299d,8,detected_all_frames,0.10,512,12,0,079165105d4c56e1612091987c08c2627049423025f74c0d4e245fb47c2ff0e3,detected,26.2932,1.0072
-2026-07-31,https://storage.googleapis.com/gdm-deepmind-com-prod-public/media/media/veo__veo-3__off-road.mp4,79a552b9406a079682440c31f14d33a10ba8e1b8b2e96425f5de70f63350299d,8,detected_all_frames,0.15,512,12,0,1c4046bcfdead138353b4e2a73339ba227bb5e544878d80c5bc6cd8427c7b00e,not_detected,25.3911,1.0578
+date,source_url,source_sha256,source_width,source_height,source_fps,duration_seconds,source_verdict,vae,noise_std,long_side,fps,seed,output_sha256,output_verdict,output_verdict_text,output_detected_range,track,session_id,stratum,psnr_db,temporal_residual_ratio
+2026-07-31,https://storage.googleapis.com/gdm-deepmind-com-prod-public/media/media/veo__veo-3__off-road.mp4,79a552b9406a079682440c31f14d33a10ba8e1b8b2e96425f5de70f63350299d,,,,8,detected_all_frames,,0.10,512,12,0,079165105d4c56e1612091987c08c2627049423025f74c0d4e245fb47c2ff0e3,detected,,,,,,26.2932,1.0072
+2026-07-31,https://storage.googleapis.com/gdm-deepmind-com-prod-public/media/media/veo__veo-3__off-road.mp4,79a552b9406a079682440c31f14d33a10ba8e1b8b2e96425f5de70f63350299d,,,,8,detected_all_frames,,0.15,512,12,0,1c4046bcfdead138353b4e2a73339ba227bb5e544878d80c5bc6cd8427c7b00e,not_detected,,,,,,25.3911,1.0578