[Mattral]

Summary

Tooling for run 2's pilot and design (no change to existing behaviour unless the new options are used).

• worldgap.data.degradations.ImageDegradation: graded, deterministic image degradations (darken, downscale, blur, noise, the last seeded per frame). Each is a frozen, JSON-describable value, so a pre-registered condition can be re-created exactly. These are simulated degradations of real footage and must be labelled so.
• extract_rollout_from_video(..., frame_transform=...): applies the transform to each decoded frame before MediaPipe and records it in metadata["frame_transform"].
• paired_landmark_error(reference, degraded): for paired data (same video, one degraded), the distance between the degraded hand landmarks and the clean detection of the same frame, in units of the clean hand's size. A left/right label swap isn't counted as error, and frames lacking a hand on either side are skipped (dropout is measured separately). It measures tracking quality, which the presence mask (a world-model input) doesn't contain. That's why it's the candidate ground truth for comparing the gap score against a trivial dropout-counting baseline: with dropout as the ground truth, that baseline is the ground truth (ρ = 1).
• scripts/run_v1_pilot.py: for one clean recording and a severity grid, reports dropout, landmark error and the trivial baselines (no_hand_fraction, presence_density), plus how well the dropout baseline alone ranks landmark error. By design it computes no gap scores or gap correlations, so severities can't be chosen by looking at the tool's performance.
• v1_pilot/ is gitignored.

Tests: tests/test_degradations.py (12):

• each degradation, and refusal of invalid ones;
• the loader hook, which applies the transform and records it;
• paired error: zero for identical rollouts, exact units, label-swap tolerance, skipped frames, unpaired shapes refused;
• a pilot-script smoke test asserting no gap or Fréchet field appears in its output.

Verification

• pytest -q: 175 passed
• ruff check .: clean; the new script is 100755
• CI green on 3.10 / 3.11 / 3.12

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