Cloth sheets falling onto each other can pass through each other

What is wrong

• Sheets dropped onto each other can end with the layer order broken: parts of an upper sheet below the sheet under it. add_cloth sheets collide with each other (and each with itself) through VBD's self contact, which should keep them apart.
• It was seen with several sheets released together over the ground, so each lands on the ones that landed before it.
• Nothing flags it: sim.health() does not read INVALID, no contact was dropped for lack of room (no SELF_CONTACT_OVERFLOW), and sim.check()["ok"] stays True.
• Sheets placed 2.5 mm apart and left to settle kept their order over the 0.15-0.24 s measured.

Number

CPU, the default cloth settings, main 03616d08c (2026-11-10). Cotton sheets of 16 x 16 vertices, 2 mm thick, 5 cm apart in height, released together over a ground plane. Measured per vertex column (the same grid vertex in every sheet) as the height difference to the sheet below; a column is crossed when that is 0 or less:

Sheets	Time reached	Crossed columns	Smallest gap	Self-contact rows used
12	1.36 s	27	-10.5 mm	24 / 46 of 32 / 64
20	0.95-0.99 s	70-108	-5.3 to -15.3 mm	24 / 40 of 32 / 64
12, placed 2.5 mm apart	0.24 s	0	2.0 mm	-
20, placed 2.5 mm apart	0.15 s	0	1.98 mm	-

• The same with the self-contact rows sized to the scene (docs/known_gaps/vbd-self-contact-buffer-overflow.md): no pair was dropped in either mode.
• The fastest particle at the end of the 12-sheet run moved at 1.05 m/s, under the 3 m/s self-contact cap (docs/known_gaps/vbd-self-contact-speed-cap.md). Speeds at the moments of impact were not recorded.

Why

• Not traced yet. Ruled out by the rows above: dropped contact pairs (the rows never filled) and contact rows sized too small (the scene-sized rows give the same crossings).
• Open: how the sheets' impacts on the sheets below interact with the motion cap and the truncation that keeps self contact penetration-free, and whether the cloth's step or iteration count changes it.

Workaround

• None verified.
• To check a scene, compare neighbouring sheets' heights yourself (record "<sheet>.points" and take, per grid vertex, the height difference to the sheet below); health will not report it.
• A stack that only needs to rest can be placed instead of dropped: sheets 2.5 mm apart kept their order over the short runs above (not checked over seconds).

Planned fix

• Trace the crossings on the 12-sheet drop (which pairs cross, at what relative speed, before or after the truncation), then fix in the cloth self-contact path. Not before the 0.1.0 tag.