离散执行器导数指标通过执行器序号控制限值

Intro Hi! I am testing order invariance for the discrete integrator with variable-width actuator inputs. The effective-metric derivative reads the target control from its control-block address but checks its limits using the actuator ordinal. ### My setup - MuJoCo source build from main at 10124d5d9dca411ec3c8988aa1e3b619103d71bb - C API, double precision - Ubuntu 24.04.5 LTS, Linux 7.0.0-28-generic x86_64 - Normal GCC 11.5.0 build and GCC 13.3.0 AddressSanitizer build ### What's happening? What did you expect? actuatorInput correctly reads d->ctrl[m->actuator_ctrladr[i]], but then uses m->actuator_ctrllimited[i] and m->actuator_ctrlrange[2*i]. Those metadata arrays are indexed by packed control slot. I expected inserting or moving a physically inactive variable-width actuator to leave the target actuator's force derivative and next state unchanged. The first reproducer builds the same three-input zero-force PID and limited affine actuator in opposite orders. With an in-range target control, both steps agree. With raw control 100, the forward path clamps both target forces to 0.5, but only the target-first derivative uses the clamped input: control=1 target-first adr=0 qvel=0.0049995000499950008 force=0.5 control=1 target-second adr=3 qvel=0.0049995000499950008 force=0.5 control=1 velocity-delta=0 control=100 target-first adr=0 qvel=0.0049995000499950008 force=0.5 control=100 target-second adr=3 qvel=0.0049504950495049506 force=0.5 control=100 velocity-delta=-4.9005000490050243e-05 The second reproducer places a zero-input DC motor before the target. A whole-library AddressSanitizer build reports: ERROR: AddressSanitizer: use-after-poison READ of size 1 #0 actuatorInput ... src/engine/engine_derivative.c:2415 #1 actuatorLenDeriv ... src/engine/engine_derivative.c:2495 #2 mjd_effActuation ... src/engine/engine_derivative.c:4088 Relevant source: https://GitHub.com/google-deepmind/mujoco/blob/10124d5d9dca411ec3c8988aa1e3b619103d71bb/src/user/user_model.cc#L3852-L3861 https://GitHub.com/google-deepmind/mujoco/blob/10124d5d9dca411ec3c8988aa1e3b619103d71bb/src/engine/engine_derivative.c#L2404-L2427 https://GitHub.com/google-deepmind/mujoco/blob/10124d5d9dca411ec3c8988aa1e3b619103d71bb/src/engine/engine_derivative.c#L4072-L4104 ### Steps for reproduction 1. Build the affected revision normally and run poc/order_invariance.cc with the commands in poc/README.md. 2. Observe equal forces but different velocities only when the target is clamped and follows the three-input actuator. 3. Build MuJoCo and poc/zero_input.cc with AddressSanitizer. 4. Observe the invalid metadata read in actuatorInput. Reproduction package: discrete-integrator-control-limit-indexing-poc.zip ### Minimal model for reproduction Both complete asset-free MJCF models are embedded in the C++ programs. The