Rename multi-segment functions to two-concept names; polish docstrings

Naming pass: rename functions whose third+ segment is redundant or
implementation-detail, sticking to the codebase's preferred
``noun_verb`` / ``verb_noun`` two-concept idiom. Renames are atomic
across definitions, callers, and tests.

  is_penned_position        →  is_penned
  modulate_speed_near_sheep →  modulate_speed
  mecanum_kinematics_step   →  mecanum_step
  policy_forward_mean       →  forward_mean

Two-concept patterns like ``velocity_to_wheels`` / ``detections_from_scan``
/ ``make_strombom_predictor`` are left alone — they're idiomatic
converters / factories that read as a single concept, and the longer
form aids grep-ability.

Docstring polish:
* ``herding/config.py`` header drops the "previously lived as a
  module-level literal" historical framing — we ship as a single
  thing, so the refactor anecdote no longer earns its keep. The
  usage examples now mention both ``HERDING_WEBOTS`` and
  ``HERDING_MEC_WEBOTS`` presets.

126 pytest cases still pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

herding/world/diffdrive.py

@@ -3,7 +3,7 @@ controllers.
 
 First-order rigid-body model — no slip, wheel-accel limits, or contact
 forces by default.  Pass ``slip_std`` and an ``rng`` to
-:func:`kinematics_step` / :func:`mecanum_kinematics_step` to add
+:func:`kinematics_step` / :func:`mecanum_step` to add
 per-wheel Gaussian speed noise for domain randomisation.
 """
 
@@ -80,7 +80,7 @@ def heading_speed_to_wheels(heading, speed_motor, h, max_wheel_omega,
 # Mecanum (4-wheel omnidirectional) kinematics
 # ---------------------------------------------------------------------------
 
-def mecanum_kinematics_step(x, y, h, w_fl, w_fr, w_rl, w_rr,
+def mecanum_step(x, y, h, w_fl, w_fr, w_rl, w_rr,
                              wheel_radius, lx, ly, dt,
                              slip_std: float = 0.0,
                              rng: Optional[np.random.Generator] = None,