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>

docs/module-internals.md

@@ -252,7 +252,35 @@ quality measurements. Neither is a watermark detector. The high-level result
 reports completed removal without a separate verification-status flag. The companion
 `scripts/video_synthid_sweep.py` imports the same engine helpers to build a
 matched control and candidate grid, preventing research and shipped
-regeneration paths from drifting. The full-clip oracle floor is
+regeneration paths from drifting.
+
+The engine's `psnr_db` is measured against the already-resized frame and before
+the encoder, so it scores the VAE round trip plus latent noise and cannot see
+the downscale, the decimation, or the codec. No in-loop metric can: the
+candidate frame is captured before it reaches the encoder pipe.
+`scripts/video_fidelity_probe.py` covers the rest by decoding the delivered file
+after muxing, upscaling it back to the source geometry, and scoring it against
+the untouched source frames. It also reports the delivered file's bitrate, so a
+fixed-crf bitrate rise cannot read as unchanged quality; because the mux copies
+source audio verbatim, that figure is a container bitrate, not a video one. The
+probe streams and accumulates the same way the engine does, so its peak memory
+does not grow with clip length. It drives the source through the engine's own
+`_iter_sampled_frames` at the source geometry rather than repeating the
+selection rule: a frame-count check cannot catch a rule that reorders frames
+without changing how many, so the rule itself has to be shared.
+
+`load_video_vae_runtime` asserts the default model's latent scaling factor
+against `VIDEO_SYNTHID_VAE_SCALING_FACTOR` and warns that no certified profile
+exists for any other model. The published `sd-vae-ft-mse` config carries no
+`scaling_factor` key, so the value is a `diffusers` class default under an
+upper-unbounded pin, and the certified profile is a perturbation-to-signal ratio
+rather than a bare `noise_std`. A library bump that moved that default would
+otherwise rescale every perturbation with a green suite. The validated factor is
+carried on `VideoVaeRuntime` and passed into encode and decode, so the gated
+value and the applied value are one measurement rather than three independent
+reads. `scripts/video_synthid_sweep.py` loads through the same function: the
+harness that produces the certified rows is the last place that should be exempt
+from the gate. The full-clip oracle floor is
 `noise_std=0.15`: on the public eight-second Veo carrier, `0.10` remained
 detected while `0.15` did not.