The seam is real: cross-chunk boundary read is NOT a null (#22/#1)

2026-08-09 ~15:2xZ. The boundary-incompatibility CPU read banked from the SEAM lit slice — exploratory, record-only, zero GPU: a pure function of five banked full-panel npz stacks. Reads produced by fontaine/scripts/boundary_incompat_results.py (one command, oracle-gated pre-data: planted compatible/offset pairs recovered exactly, degenerate same-frame overlap reads exactly 0, NaN poison on invalid steps leaks nowhere, dt=48 hand fixture to float64, 6 abort branches). No decision rule was registered and none is applied — this read was priced as “a null closes the #22 bridging direction for our stack”; the null did not materialize.

TLDR

Our chunks disagree at the seam, a lot, for every policy we have banked — and the disagreement decomposes cleanly. For same-episode panel frames dt < 50 ticks apart, the earlier chunk’s tail and the later chunk’s head predict the same actions (truth overlaps agree byte-exactly on all 13,693 pairs — asserted, not assumed). The two predictions disagree by MAE ≈ 6.3–8.4 (norm. action units) pooled over all dt — comparable to each model’s own error on the same overlap (D/err ≈ 1.1–1.27), and the executed-trajectory jump at the switch point is 11–14× the typical per-step motion while chunks stay smooth inside (the SDN within-chunk null replicated). Smooth within, jerky between: SEAM’s problem statement, confirmed on our stack at k4l2 geometry.

The dt→0 intercept (dt ≤ 5, observations nearly identical) splits the cause:

Fresh-noise flow — the deployment condition — carries a ~3.3-unit pure noise/mode term above the deterministic baseline: two draws from nearly the same observation land ~6 units apart. Sharing the noise across frames (ticket33) deletes that term entirely and lands below greedy AR — the most seam-consistent policy we have banked. That is direct, free evidence for the SEAM/PAINT-family premise: cross-chunk noise coupling is the cheap lever on the seam.

The read, exactly

Panel plans place ~6 frames per episode (4 core + 2 labeled); 13,693 pairs sit 1–49 ticks apart, dt near-uniform. For a pair (dt), the early chunk’s steps [dt:50) and the late chunk’s steps [0:50−dt) cover the same wall-clock actions. Per pair, valid-masked:

• D = mean |early_tail − late_head| over the overlap;
• anchors: each side’s MAE vs truth on the same overlap (D/err), and within-chunk step size W = mean |a[t+1] − a[t]| (the SDN anchor);
• J = |late_head[0] − early_tail[0]|, the switch cost the executed trajectory would pay at the seam.

Pooled mean + bootstrap CI95 + leave-one-repo-out, the banked box-batch conventions. Full numbers (per-dt-bin curves, LORO, state-copy references): reports/analysis__boundary_incompat_panels.json.

Headline pooled rows (all 13,693 pairs):

State-copy’s D (10.65 pooled — it is exactly |state(f1) − state(f2)|) is the scene-motion scale: every model’s seam disagreement sits well under “the scene moved”, but far above “the plans agree”.

What this does and does not say

• The dt-curve slope is partly a horizon effect, not all observation drift. The early chunk’s overlap steps are its far-horizon predictions (err 7.8–8.7), the late chunk’s are near-horizon (err 3.6–4.5) — regression-toward-the-mean at long horizon inflates D as dt grows. The intercept is the clean statistic: at dt ≤ 5 both sides predict at nearly matched horizons from nearly the same observation, and the fresh-noise flow still disagrees with itself by 6 units.
• AR is not seam-clean either. 2.7 units of disagreement at nearly-identical observations for a deterministic greedy decode means the argmax plan itself is sensitive to tick-level observation change (and the 40k→60k trunk did not shrink it). Noise is the biggest seam term, not the only one.
• Open-loop proxy caveat, carried loud. In deployment the next chunk is generated from the observation after executing the previous one; our pairs condition both sides on ground-truth observations. This read prices the disagreement term SEAM targets, not closed-loop jerk itself — the offline panel can price the problem but must never be asked to validate a fix (the fix is #16-gated by construction).
• Record-only. No decision rule fires. Any SEAM/PAINT-class arm, or a noise-coupling deployment policy (shared/slow-varying ticket across consecutive chunks), needs its own pre-registration and a rig bench to score on.

What it changes

1. #22 stays alive and gains a measured target. The direction this read could have closed at zero cost is instead confirmed: seam disagreement ~1.1–1.3× model error, boundary jump 11–14× step motion. Arm order unchanged (measure naive-switch cost → HAS → SEAM → PAINT → A2C2 → DEFLECT-class), still parked on #16.
2. Noise coupling is evidenced, not just imported. The ticket33 column is an accidental ablation the GoldenTicket bank paid for already: sharing noise across frames removes the entire noise-induced seam term (6.04 → 2.07 at dt≤5). A deployment policy as dumb as “reuse yesterday’s ε” beats per-chunk fresh noise at the seam — SEAM/PAINT get the same effect while keeping per-chunk diversity.
3. #1 gets a cross-chunk data point the within-chunk SDN null could not see: draw dispersion that is invisible to per-draw smoothness statistics shows up as 6 units of plan disagreement at matched observations.