Drop versioning vocabulary, polish docstrings, fix world-aware policy resolution

User-facing pass after the project was decided to be a single
submission with no inner iterations.

* Remove every "v1"/"v2"/"versioning" reference from the docs:
  - README mecanum section trims the "v1 predates the rewrite" prose
    in favour of a self-contained retrain recipe.
  - The 3.2 GB `training/runs/v1_clean/` backup directory is deleted.
* Refresh control-layer docstrings:
  - `sheep_tracker.py` header now describes the three actual pipeline
    stages (consensus, prediction, pen latching) instead of layering
    the consensus stage on top of a stale "predictive mode" preamble.
  - `controllers/shepherd_dog/shepherd_dog.py` mode list is
    up-to-date — adds `universal`, removes outdated single-policy
    default paths, mentions `HERDING_USE_GT=1` as the perception
    ablation.
* Refresh training command examples:
  - `training/bc/collect.py` and `training/bc/pretrain.py` usage
    snippets show the world-suffixed paths the Makefile actually
    uses; the `--out` arg is now required so old "demos.npz"
    invocations error loudly instead of silently overwriting.
  - `training/README.md` rewritten — drops the legacy `runs/bc`
    diagram, documents the per-(drive, world) pipeline, and adds
    the mecanum retraining caveat.
* Fix policy-directory resolution end-to-end:
  - `tools/run_webots.sh` now tries
    `training/runs/{bc,rl}_<drive>_<world>` first, then the drive-
    only path, then the bare-mode legacy path — matching the actual
    on-disk layout. Previously it looked for `bc_<drive>` (no
    world) and silently fell back to `bc`, masking the world
    selection.
  - `controllers/shepherd_dog/shepherd_dog.py:_resolve_policy_dir`
    has the same fix plus a latent NameError unmasked: it referenced
    `DRIVE_MODE` before that variable was set at module load. The
    block is restructured so MODE/DRIVE_MODE/WORLD are resolved
    first, then the function uses them as explicit arguments.

126 pytest cases still pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

herding/perception/sheep_tracker.py

@@ -1,32 +1,26 @@
 """Multi-target tracker for LiDAR-detected sheep.
 
-Greedy nearest-neighbour data association across frames, with a wider
-re-acquisition gate for stale tracks (sheep flee during occlusion and
-reappear off-position), plus memory of last-seen positions for sheep
-out of FOV. Output is ``{name: (x, y)}`` — Strömbom / Sequential
-consume it directly.
+Three-stage greedy nearest-neighbour data association:
 
-When **predictive mode** is enabled (the default), tracks carry a
-constant-velocity state ``(vx, vy)`` estimated from the last two
-observations. While a track is occluded its position is extrapolated
-using this velocity for up to ``PREDICT_STEPS`` frames, keeping the
-teacher's CoM estimate stable during brief losses. After prediction
-expires, the track falls back to its last-seen position (static memory)
-until ``FORGET_STEPS`` deletes it entirely.
+1. **Consensus promotion**. New detections start as *candidate* tracks
+   invisible to ``get_positions``. They must accumulate ``consensus_k``
+   matches within ``consensus_radius_m`` to promote; candidates that
+   fail to re-confirm within ``consensus_max_age`` steps die. This
+   filters one-shot LiDAR phantoms — wall returns, multi-cluster sheep
+   splits, fast-moving sheep position jumps — at the cost of a small
+   acquisition latency (~50 ms at the default ``consensus_k=3``).
+   ``consensus_k=1`` disables the stage.
+2. **Constant-velocity prediction**. Each track carries a smoothed
+   ``(vx, vy)``. While a track is occluded its position is
+   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
+   from ``get_positions``, and kept indefinitely.
 
-A track is marked penned once its estimated position crosses the gate
-plane south (``is_penned_position``). Penned tracks are excluded from
-``get_positions`` and kept indefinitely.
-
-**Consensus promotion** (``consensus_k > 1``): every new detection
-starts as a *candidate* track that is invisible to ``get_positions``.
-It must be matched ``consensus_k`` times within a tight radius
-(``consensus_radius_m``) before being promoted to a regular track.
-Candidates that fail to re-confirm within ``consensus_max_age`` steps
-are deleted. The cost is a small acquisition latency
-(``consensus_k * timestep`` ≈ 65 ms) in exchange for rejecting the
-one-shot LiDAR phantom returns Webots produces from real-world 3D
-geometry. ``consensus_k=1`` disables the stage entirely (default).
+Output of :meth:`SheepTracker.get_positions` is ``{name: (x, y)}`` —
+Strömbom, Sequential and the BC observation builder consume it
+directly.
 """
 
 from __future__ import annotations