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>

controllers/sheep/sheep.py

@@ -27,7 +27,7 @@ from herding.world.diffdrive import heading_speed_to_wheels
 from herding.world.flocking_sim import MAX_SPEED, compute_heading_speed
 from herding.world.geometry import (
     SHEEP_MAX_WHEEL_OMEGA,
-    is_penned_position,
+    is_penned,
 )
 
 
@@ -92,7 +92,7 @@ while robot.step(timestep) != -1:
     pos = gps.getValues()
     x, y = pos[0], pos[1]
 
-    if not penned and is_penned_position(x, y):
+    if not penned and is_penned(x, y):
         penned = True
         paint_pink()