Fix muscle FLV documentation to match current implementation in engine_util_misc

[zchJo]

Problem

While fitting FLV curve parameters for musculoskeletal modeling, I noticed a discrepancy between the forces computed at runtime by MuJoCo and the reference MATLAB implementation in doc/_static/FLV.m .

In particular, using FLV.m as the reference for parameter fitting produced curves that differed from the forces computed at runtime by MuJoCo. After comparing the documentation script with mju_muscleGainLength and mju_muscleBias, the mismatch appears to be stale documentation/assets rather than a simulator bug.

Summary

This PR updates the muscle FLV documentation assets so they match the current MuJoCo muscle actuator implementation.

The previous doc/_static/FLV.m script still used an older force-length/passive-force curve:

• active force-length included an extra short-length 0.15 bump that is not present in mju_muscleGainLength;
• passive force-length used a cubic/linear curve that no longer matches mju_muscleBias;
• the generated musclemodel figures and the fpmax XML reference text reflected that older passive curve.

The current implementation in src/engine/engine_util_misc.c uses one quadratic active force-length bump and a passive force curve that is half-quadratic up to (1 + lmax) / 2, then linear beyond that point. MuJoCo Warp/MJX mirrors this logic.

Changes

• Updated doc/_static/FLV.m to match mju_muscleGainLength and mju_muscleBias in src/engine/engine_util_misc.c.
• Kept the MATLAB figure style, with slightly larger labels, bold activation annotations, and tighter subplot spacing.
• Regenerated doc/images/modeling/musclemodel.png and doc/images/modeling/musclemodel_dark.png with MATLAB R2024b at the original pixel dimensions.
• Updated doc/XMLreference.rst so fpmax is described as a passive force scaling parameter instead of passive force exactly at lmax.

Testing

• Ran git diff --check.
• Visually inspected the regenerated light and dark muscle model figures.
• Verified both regenerated PNGs are 1435x1272 pixels.
• Verified the corrected default passive curve key points: FP(1) = 0, FP((1 + lmax) / 2) = 0.5 * fpmax, FP(lmax) = 1.5 * fpmax.

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