fix: correct unreasonable terrain level update

[linden713]

Fixes an unreasonable terrain-difficulty downgrade behavior in the terrain curriculum. The community has reported similar issues where the curriculum (which measures distance from the env origin) can fail in some cases. Ref: IsaacLab discussion.

What I changed

• Add params to control upgrade and downgrade
• Prevent downgrade when the episode end with timed out.
• Consider only the last command window when estimating expected distance. Episode length is 20s and commands are resampled every 10s. This prevents agents that are slow early but fast later from being unfairly downgraded.

Why

Even for flat terrains, the level never rises above 1.5, which is unreasonable and prevents normal progression.

This image shows terrain levels on flat terrain — it should reach 6, but the original function is capped at 1.5. Staying at 1.5 on flat terrain is unreasonable; reaching 6 is normal and expected.

This image shows results from my go2 experiments on mixed terrains (stairs, slopes, etc.) using fixed terrain level function. The curriculum reaches 6, which matches expected behavior.