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>

tests/test_geometry.py

@@ -5,7 +5,7 @@ import math
 from herding.world.geometry import (
     FIELD_X, FIELD_Y, GATE_X, GATE_Y, MAX_SHEEP, PEN_ENTRY, PEN_X, PEN_Y,
     distance_to_pen_entry, in_field, in_gate_corridor, in_pen,
-    is_penned_position,
+    is_penned,
 )
 
 
@@ -44,23 +44,23 @@ def test_in_gate_corridor():
     assert not in_gate_corridor(5.0, -18.0)
 
 
-def test_is_penned_position_latches_below_gate():
+def test_is_penned_latches_below_gate():
     # In the gate column and south of the gate plane → penned.
-    assert is_penned_position(11.5, -15.0)
-    assert is_penned_position(10.5, -18.0)
-    assert is_penned_position(12.5, -22.0)
+    assert is_penned(11.5, -15.0)
+    assert is_penned(10.5, -18.0)
+    assert is_penned(12.5, -22.0)
     # Above the gate plane → not yet.
-    assert not is_penned_position(11.5, -14.9)
+    assert not is_penned(11.5, -14.9)
     # Outside the gate column → not penned even if south.
-    assert not is_penned_position(0.0, -16.0)
-    assert not is_penned_position(14.0, -16.0)
+    assert not is_penned(0.0, -16.0)
+    assert not is_penned(14.0, -16.0)
 
 
-def test_is_penned_position_latch_margin():
+def test_is_penned_latch_margin():
     # Slight tolerance on the gate column.
-    assert is_penned_position(9.9, -15.5)
-    assert is_penned_position(13.1, -15.5)
-    assert not is_penned_position(9.7, -15.5)
+    assert is_penned(9.9, -15.5)
+    assert is_penned(13.1, -15.5)
+    assert not is_penned(9.7, -15.5)
 
 
 def test_distance_to_pen_entry():