Shipped robots left with no targets move away from their start angles

What is wrong. A shipped robot added with scene.add_robot(...) and stepped with no targets does not keep the joint angles it starts at. Two causes, one per robot family: - The start is outside a joint range (the Franka arms). alloy.build starts a robot at joint angles 0, and on these arms 0 is outside the range of joint 4 (and of joint 6 on the FR3). The joint limits push those joints into range in the first 0.2 s, and the servos, whose default target is also 0, then press them against the limit. - Position servos sag under gravity (the legged robots, the humanoids' arms). A position servo holds a joint by its error: under a steady load the joint settles torque / kp away from its target. With the source files' gains that is up to a few degrees. The health checks do not flag either: sim.check()["ok"] is True in every case below.

The number. Largest |q(2 s) - q(0)| per joint, one world, CPU, 10 ms control steps, no targets; the same to four digits on the MuJoCo C and the MuJoCo Warp routes (and with or without the C extension, OSS-17).

pack	joint	moves	cause
emb_e1_fr3, emb_e4_fr3_bimanual (each arm), emb_e5_fr3_duo (each arm)	joint 6	+0.439 rad	start 0 below its range (0.44 to 4.62 rad): pushed to the limit, 0.43 rad of it in 0.1 s
same	joint 4	-0.117 rad	start 0 above its range (-3.08 to -0.117 rad)
emb_e2_panda	joint 4	-0.070 rad	start 0 above its range (-3.07 to -0.070 rad)
emb_e8_go2	rear / front calves	-0.147 / -0.119 rad	the 15 kg body on kp 50 servos (Menagerie's gains): the base settles from 0.288 to 0.264 m
emb_e8_go2	thighs	-0.033 rad	same
emb_e7_h1_2	elbows	0.045 rad	the arms' weight on kp 80 servos (the source profile's): torque / kp = 0.046 rad
emb_e7_h1_2	shoulder pitch, wrist pitch	0.028, 0.011 rad	same, kp 120 and 40 (torque / kp 0.032, 0.011 rad); the passive thumb joints follow their mimic parents (0.028 rad)
emb_e6_g1	ankle pitch, waist pitch, knees	0.022, 0.020, 0.017 rad	kp 500 legs and waist under the body
The FR3 and Panda arms' other joints move under 0.002 rad.

Why. The Franka packs keep the sources' joint zero, which is not inside every joint range; the start pose is alloy.build's rule (joint angles 0), not the files' init keyframe. The sag is the sources' servo gains: a real robot's controller adds a gravity feed-forward that these position servos do not have.

Workaround. - Franka arms: send targets inside the joint ranges, ramped from the start (for example to the pack's init keyframe, 0 0 0 -1.571 0 1.571 -0.785 on the FR3), before anything that depends on the arm's pose. - Sag: read the joint state rather than assuming the target, or aim the target past the wanted angle by the sag (the torque the joint carries divided by its kp, in the pack's scene.xml).

Planned fix. After the release tag the FR3 and Panda packs get a start inside their joint ranges (the joint zero moved to the home pose, as for the Go2: docs/known_gaps/fr3-link5-link7-exclude.md). The sag is the sources' gains and stays; no gravity feed-forward is planned.