Stage-2 sign probe: the escalation branch fired — 3 of 4 reference populations are not sign-consistent

2026-08-05 ~23:5xZ. Execution of the stage-2 pre-registration, instrument frozen before any probe code existed. CPU-only (~35 min on nice-19 workers beside the live box batch and draws chain). Probe: probes/probe_sign_convention_stage2.py; full numbers in ~/sign_stage2_results.json. No candidate cell was opened and no mirror verdict ships — this is the pre-registered invalid-by-population branch, and per the pre-reg the population diversity itself is the finding that escalates to the owner.

What happened, in one paragraph

The instrument requires, per target dim, a 15-repo so100 reference population whose optical-flow-vs-joint-velocity sign agrees ≥ 80% (≥ 12/15) with median |ρ| ≥ 0.2. Three of the four populations FAILED that gate — wrist_roll 9/15 (median ρ +0.16), wrist_flex 10/15 (−0.13), shoulder_lift 9/15 (−0.46 but only 60% agreement). Only shoulder_pan passed (13/15, median −0.24). The synthetic-flip hard gate therefore could only run on the t_x family, where it passed cleanly (original NORMAL with bootstrap mass 1.000, doctored MIRRORED with mass 1.000, ρ ∓0.887 on 00ri/so100_battery) — so the mechanism works where the population premise holds. With ω and t_y untestable, the pre-reg’s hard gate fails overall and all three candidate cells (dishTidyUp_anomaly wrist_flex, groceriesSorting_expert wrist_roll, aractingi shoulder_lift) stay closed. No verdicts, no repair-arm eligibility, no falsification.

Why the populations split: it’s camera geometry, not (necessarily) joint conventions

The pre-reg said a failed population gate “would mean sign conventions vary corpus-wide, a bigger finding than three repos.” The diagnostic cuts point at a sharper — and more repairable — mechanism:

• The sign of an image-plane flow statistic depends on which camera you read it from. On shoulder_lift, the two cameras of the same repo disagree in sign in 11/15 reference repos (ego ρ and other-cam ρ are near mirror images: e.g. AkibaGeek +0.83 ego vs −0.77 other). A wrist-mounted camera sees the world translate opposite to how a fixed front camera sees the arm move. The per-repo |ρ| values are large (up to 0.85) — the instrument has power — but the population pools over repos whose camera geometry differs, so the signs never converge.
• The flow-based ego-cam rule is too weak to carry that conditioning. It had NO-MARGIN (< 0.15) on roughly half the repos (8/15 on shoulder_lift), i.e. near-coin-flip camera picks exactly where the sign depends on the pick. Restricting to margin-confident repos improves agreement (wrist_roll 5/7, wrist_flex 6/8, shoulder_lift 5/8) but stays below the 80% gate.
• ω (wrist_roll) is additionally underpowered: only 2/15 reference repos reach |ρ| ≥ 0.3. Image-plane rotation about the frame center only tracks wrist roll when the reading camera looks along the roll axis — i.e. it is a wrist-cam statistic, and most ego picks were fixed cams.
• Where the geometry is homogeneous the instrument behaves: shoulder_pan’s t_x (most so100 uploaders put a fixed cam facing the arm; pan sweeps horizontal flow) passed at 13/15, and its oracle hit mass 1.000 in both directions.

So the honest read is not “so100 joint sign conventions vary corpus-wide”; it is “image-space sign references vary with camera mounting, which is unlabeled and heterogeneous — the reference-population premise as frozen was wrong for 3 of 4 dims.” Stage 1’s three mirror candidates remain exactly what they were: unresolved screening leads.

The way through (for a stage-2b amendment, before any new run)

The corpus already carries the missing conditioning variable: meta/camera_kinds.json — the VLM camera-labeling pass (2026-08-02, opus-5, per-cam wrist/front/side/top votes, mostly unanimous). A stage-2b would re-pool reference populations per (dim, camera kind) — t_y from front cams only, ω from wrist cams only, and the ego-cam rule replaced (or gated) by the label — leaving every other frozen constant unchanged. That removes the geometry confound the data says is the blocker, at the cost of a smaller per-cell reference pool. If the owner wants the candidate cells settled, that is the amendment to pre-register; it reuses today’s flow cache (the expensive decode is done and keyed by repo).

Pre-declared consequences, applied

• Candidate cells: not opened (hard gate failed) — per pre-reg, “no candidate verdicts, and the diversity escalates to the owner.”
• Ideas #13 stays screening; the repair arm is neither eligible nor dead — stage 2 as frozen cannot adjudicate it.
• Controls: never read (cells phase never ran); the instrument-fault clause was not triggered — the failure is upstream, in the population premise.
• The stream-consistency reads (step 7) were deliberately not run on candidates: they accompany verdicts, and running them outside a verdict frame would be unregistered peeking.

Instrument facts (for the record)

15-repo populations selected lexicographically from 374–442 eligible so100 repos per dim (≥ 8 stage-1 panel frames, MAE ratio ≤ 2.0 on the dim, ≥ 30 isolated pairs at 2.0× dominance — no repo needed the 1.5× relax); 40–400 pairs per repo; Farneback at 320×240 with the frozen params; episode-bootstrap 1000 draws, SeedSequence(13). Flow cache: 38 repos under ~/sign_stage2_cache/. The oracle’s doctored read is a true end-to-end negation (pair selection invariant, verdict machinery identical), not a sign flip of the summary statistic.