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/geometry.py

@@ -171,7 +171,7 @@ def in_gate_corridor(x: float, y: float, margin: float = 0.0) -> bool:
             and PEN_Y[0] - margin <= y <= GATE_Y + margin)
 
 
-def is_penned_position(x: float, y: float, latch_margin: float = 0.2) -> bool:
+def is_penned(x: float, y: float, latch_margin: float = 0.2) -> bool:
     """True iff (x, y) is in the gate column and south of the gate line."""
     return (GATE_X[0] - latch_margin <= x <= GATE_X[1] + latch_margin
             and y <= GATE_Y)