A small body resting on a coarse soft solid can sink into one surface particle past health's limit

What is wrong

• A soft solid (soft_vbd) touches rigid bodies through one particle per mesh vertex. The particle radius r is 1/5 of the tet mesh's shortest cell edge. A 15 x 15 x 5 cm pad has 12.5 mm cells and r = 2.5 mm, so its surface particles stand 7.5 mm apart. A body smaller than a few cells can come to rest on a single particle.
• Health's limit for a soft contact is r (half of 2r, the thinner part), and a deeper contact held for 0.1 s fails the world (PEN_FAIL). In sample-sweep's drop, a steel ball came to rest 3.36 mm into one particle, and the world froze 0.1 s after it settled.
• Every landing touches 3.4-3.7 mm for a moment (DEGRADED, PEN_WARN). Whether the body then rests that deep depends on where and how it settles, so the same drop at another dt or start height can pass.
• A soft solid placed flush on its support starts one radius inside it. Alone on the ground, such a pad is thrown 3.6 cm up within 0.09 s, and nothing is flagged.

Number

One L4, main bd9e6b412 (rc6 steps the same code). Sample-sweep's ab_drop4 panel sil scene without its render: a static worktop (top at z 0), a 15 x 15 x 5 cm silicone_40a pad (845 particles, r 2.5 mm) and a 27 mm steel_carbon ball (80 g) dropped 15 cm onto the pad's top. Scripts: development probes, not shipped.

pad	dt	result
flush on the worktop (pos z 0.025)	0.01	INVALID at 0.536 s: the ball rests 3.36 mm into one particle
flush	0.005	check ok; the ball rests 0.2-0.4 mm deep
lifted one radius (pos z 0.0275)	0.01	check ok; 1.2 mm for a moment at a landing, then 0.2-0.3 mm
lifted one radius	0.005	check ok; 3.7 mm and 1.5 mm for moments at landings

• In the frozen run the ball sits on the pad's top-centre particle. The next particles are 3.7-4.3 mm from touching it, so that one particle carries the ball's 0.79 N at 3.36 mm. Rolling over the same particle in the lifted run, the ball was 0.2 mm deep.
• The ball bounces several times before it settles (soft-solid-bounce.md), so the runs settle in different places.
• The flush pad alone, at dt 0.005: its bottom is at 5 mm after one step, 3.6 cm at 0.09 s, then it falls back.

Why

• Contact with a soft solid is vertex-only (alloy/scene/emitters/deformable.py, soft_particle_radius): the surface between vertices does not push back. A body larger than the gaps cannot pass through, but it can balance on one vertex.
• Health measures a soft contact against r, the particle's own radius.
• Not traced: why one particle holds the ball 3.36 mm deep in the frozen run, against 0.2 mm when the ball rolls over it.
• A VBD solid rests one radius above what it touches, so a solid placed flush starts that far inside its support, and the first steps push it out.

Workaround

• Place a soft solid one particle radius above its support (its pos z + r), not flush.
• Under a small body, use a finer tet mesh (the object card's budget.tets sets the cell), so several particles carry the body.
• At dt=0.005 the same drop passed. Always read sim.check()["ok"] after a body settles on a soft solid.

Planned fix

• Contact on a soft solid's surface faces, not only its vertices (as cloth's full-surface contacts do), or a surface-particle radius sized to the gaps.
• A start-overlap check for soft and cloth vertices (START_PEN today judges rigid contacts only), so a flush placement is reported at build.