Buffer has no second attractor: the flip is an episode, and PVD feels the coil

[YesterdaysLemon]

Closes #195.

The map

The scorecard's buffer row said "one seed in five settles into a ~0.33 Hz whole-body standing flip." Watching instead of sampling (tools/buffer_basin.py, 15 s windows over long swims):

• 12 seeds × 120 s: eleven swim cleanly at 0.87 Hz / TWI ~0.75; seed 0 alone shows a deep excursion (TWI 0.73 → 0.08 across 75–120 s at unchanged frequency); two seeds show single-window dips and recover at once.
• Seed 0 × 300 s: the excursion ends by itself — TWI back to 0.67 by 135 s, clean for the next three minutes.
• The pre-PVD model (591af4e) shows the same shape, different timing: episodes at 45–60, 105–120, 165–180 s (freq dipping to 0.33/0.20), every one self-ending. The 2026-08-25 scorecard reading was the 45–60 s episode straddled by its own 46–86 s analyse window — a snapshot of a spell, reported as a stationary state.

Buffer has no second attractor at shipped defaults. It has a recurrent ~15 s coil-up excursion the animal always exits.

The better find

Current and pre-PVD buffer trajectories are bit-identical for 11,000 steps and then fork at t≈55 s; the bisect lands on the PVD merge (#205). Cause: the contact total PVD reads includes Body.self_contact_force, and a swimming animal's deep coil presses its own far side at whole-body totals of 3.6–8.3 — past pvd_threshold = 3.0. A hard coil is now a nociceptive event. Biologically defensible (PVD tiles the body wall; harsh body contact is harsh body contact), and now on the record in the PVD dossier rather than implicit in a trajectory diff. Agar and viscous crawls never self-contact at these totals — which is exactly why every determinism pin stayed green while buffer moved.

What changed

• tools/buffer_basin.py — the instrument, with THE RECORD.
• Scorecard's bimodality note (print + doc) now describes the episode instead of asserting a basin.
• PVD calibration dossier (params.py) carries the self-contact regime, flagged as a mechanism decision with evidence if self-touch should ever be excluded.
• NEXT.md item 1's buffer protocol guard points at the map.

No model change — instrument, record, and corrected prose only.

🤖 Generated with Claude Code

https://claude.ai/code/session_01CBroMXcYdfkyb5buJPcKvL

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