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/perception/sheep_tracker.py

@@ -15,7 +15,7 @@ Three-stage greedy nearest-neighbour data association:
    extrapolated for up to ``PREDICT_STEPS`` frames, then falls back to
    last-seen static memory until ``FORGET_STEPS`` deletes it.
 3. **Pen latching**. A track whose estimated position crosses the gate
-   plane south of ``is_penned_position`` is marked penned, excluded
+   plane south of ``is_penned`` is marked penned, excluded
    from ``get_positions``, and kept indefinitely.
 
 Output of :meth:`SheepTracker.get_positions` is ``{name: (x, y)}`` —
@@ -31,7 +31,7 @@ from typing import TYPE_CHECKING
 if TYPE_CHECKING:
     from herding.config import TrackerConfig
 
-from herding.world.geometry import MAX_SHEEP, in_pen, is_penned_position
+from herding.world.geometry import MAX_SHEEP, in_pen, is_penned
 
 
 GATE_M = 2.5              # m — primary NN gate (recently observed tracks)
@@ -356,10 +356,10 @@ class SheepTracker:
         at y ≈ -15) from being permanently latched as penned tracks.
         """
         from herding.world.geometry import GATE_Y
-        # Apply depth threshold to both in_pen and is_penned_position so
+        # Apply depth threshold to both in_pen and is_penned so
         # that any position in the gate column must clear GATE_Y - depth.
         threshold = GATE_Y - self._pen_latch_depth
-        return (in_pen(x, y) or is_penned_position(x, y)) and y <= threshold
+        return (in_pen(x, y) or is_penned(x, y)) and y <= threshold
 
     def get_positions(self, min_freshness: int | None = None) -> dict[str, tuple[float, float]]:
         """Promoted (non-candidate, non-penned) tracks as ``{name: (x, y)}``.