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/control/modulation.py

@@ -1,6 +1,6 @@
 """Shared action post-processing.
 
-Every dog mode routes its action through ``modulate_speed_near_sheep``
+Every dog mode routes its action through ``modulate_speed``
 so the magnitude is reduced near sheep — direction (intent) is
 preserved.
 """
@@ -14,7 +14,7 @@ SLOW_NEAR_SHEEP = 2.5  # m — distance below which action norm is scaled down
 MIN_SPEED = 0.30       # action norm at zero distance
 
 
-def modulate_speed_near_sheep(
+def modulate_speed(
     vx: float, vy: float,
     dog_xy: tuple[float, float],
     sheep_positions,