Per-sheep pen-time metrics, seed support, make webots → menu

* `controllers/shepherd_dog/shepherd_dog.py`
  - Tracks the first step at which each sheep crosses the gate; on
    auto-finish (all sheep penned) prints a `[results]` summary
    block: mode/drive/world/lidar/dogs/seed line, total simulated
    time, per-sheep penning order with absolute step + seconds since
    sim start, and the gate spread between the first and last
    penning.
  - Reads `HERDING_SEED` (env / runtime cfg) and seeds the
    controller's RNG when set. Empty = time-based default = old
    non-deterministic behaviour.
* `controllers/sheep/sheep.py` reads `HERDING_SEED` the same way
  (loading `herding_runtime.cfg` itself so it works even when
  Webots strips env vars) and seeds Python's RNG XOR'd with the
  sheep's name hash, so a fixed seed gives a reproducible flock
  trajectory without all sheep starting from identical wander state.
* `tools/run_webots.sh` writes `HERDING_SEED` into the runtime cfg
  (empty when unset so existing scripts keep their stochastic
  behaviour).
* `tools/webots_menu.sh` gains a Seed prompt (random / fixed
  integer); the launch summary box shows the choice next to the
  perception row.
* `Makefile`
  - `make webots`  now fires the interactive picker (replacing the
    old positional invocation).
  - `make webots_quick MODE=… DRIVE=… WORLD=… N=…` is the old
    positional path, kept for batch / scripted use.

Smoke-tested: menu renders Mode → Drive → World → LiDAR → Dogs
→ Sheep → Perception → Seed → Headless prompts and shows the
selected Seed value in the launch summary. 126 pytest cases still
pass.

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

tools/run_webots.sh

@@ -232,6 +232,7 @@ HERDING_LIDAR=$LIDAR_VARIANT
 HERDING_NDOGS=$NDOGS
 HERDING_AXIS_LEAK=${HERDING_AXIS_LEAK:-0.3}
 HERDING_USE_GT=${HERDING_USE_GT:-0}
+HERDING_SEED=${HERDING_SEED:-}
 EOF
 
 export HERDING_MODE="$MODE"