Dual-shepherd axis-split (HERDING_NDOGS=2)

The launcher can now spawn two `ShepherdDog` robots, each masked to a
single axis of motion, so the herding workload is split orthogonally.

Mechanic:
* `HERDING_NDOGS=2` (default 1) tells `tools/run_webots.sh` to replace
  the single-dog node in the generated test world with two copies:
  - `ShepherdDogX` at (-4, -10), `customData "axis=x"`
  - `ShepherdDogY` at (+4, -10), `customData "axis=y"`
  Each spawn position sits south of the field interior so the pair
  doesn't collide with starting sheep.
* `controllers/shepherd_dog/shepherd_dog.py` reads `getCustomData()`
  at startup; when `axis=x|y` it zeroes the off-axis component of every
  action *after* speed modulation and *before* EMA smoothing. With
  `customData` empty the controller behaves identically to single-dog
  mode, so all existing launches are unaffected.
* The dog's emitter line now carries the robot's name
  (`dog:ShepherdDogX:x:y`), and `controllers/sheep/sheep.py` keeps a
  `dogs` dict keyed by name, picking the closest one each step for
  its flee target. Single-dog runs still use the legacy two-field
  `dog:x:y` format thanks to a length check.
* `HERDING_NDOGS` is written into `herding_runtime.cfg` and exported
  to subprocesses so future tooling can read it.

Verified behaviour in Webots smoke tests (HERDING_NDOGS=2, strombom,
diff/field, 5 sheep): both dogs spawn with the expected names and
axis tags, the dual-dog status print appears, each dog acts only on
its assigned axis early in the trial, and the masking is internally
consistent. The pair stalls before penning under pure axis-split
because each dog reaches its drive standoff and then has only one
degree of freedom — useful research finding for the write-up;
coordination strategy (shared CoM, role-switching, etc.) is future
work.

126 pytest cases still pass.

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

tools/run_webots.sh

@@ -161,11 +161,62 @@ for i in $(seq $((N+1)) 10); do
     sed -i "s|^Sheep .* \"sheep${i}\".*|# &|" "$DST"
 done
 
+# Dual-dog axis split. When HERDING_NDOGS=2 the launcher replaces the
+# single dog node in the world with two named dogs whose customData
+# carries the axis assignment (x or y); the controller masks the
+# off-axis component of every action.
+NDOGS="${HERDING_NDOGS:-1}"
+if [[ "$NDOGS" != "1" && "$NDOGS" != "2" ]]; then
+    echo "HERDING_NDOGS must be 1 or 2, got '$NDOGS'" >&2; exit 1
+fi
+if [[ "$NDOGS" == "2" ]]; then
+    DOG_NODE_NAME="ShepherdDog"
+    if [[ "$DRIVE" == "mecanum" ]]; then
+        DOG_NODE_NAME="ShepherdDogMecanum"
+    elif [[ "$LIDAR_VARIANT" == "360" ]]; then
+        DOG_NODE_NAME="ShepherdDog360"
+    fi
+    python3 - "$DST" "$DOG_NODE_NAME" <<'PY'
+import re, sys
+path, node = sys.argv[1], sys.argv[2]
+with open(path) as f:
+    txt = f.read()
+# Match the single existing dog block from "ShepherdDog{,360,Mecanum} {"
+# up to its closing "}" on a line by itself.
+pattern = re.compile(rf"^{re.escape(node)} \{{\n(.*?\n)^\}}\n", re.MULTILINE | re.DOTALL)
+m = pattern.search(txt)
+if m is None:
+    sys.exit(f"[run_webots] could not locate single-dog block ({node}) for split")
+two_dogs = (
+    f"{node} {{\n"
+    f"  translation -4 -10 0.5\n"
+    f"  rotation 0 0 1 1.5708\n"
+    f'  name "ShepherdDogX"\n'
+    f'  customData "axis=x"\n'
+    f'  controller "shepherd_dog"\n'
+    f"}}\n"
+    f"{node} {{\n"
+    f"  translation 4 -10 0.5\n"
+    f"  rotation 0 0 1 1.5708\n"
+    f'  name "ShepherdDogY"\n'
+    f'  customData "axis=y"\n'
+    f'  controller "shepherd_dog"\n'
+    f"}}\n"
+)
+with open(path, 'w') as f:
+    f.write(txt[:m.start()] + two_dogs + txt[m.end():])
+PY
+fi
+export HERDING_NDOGS="$NDOGS"
+
 active=$(grep -c '^Sheep' "$DST" || true)
+ndog=$(grep -cE '^(ShepherdDog|ShepherdDog360|ShepherdDogMecanum) \{' "$DST" || true)
 echo "------------------------------------------------------------"
 echo "World      : $DST"
 echo "Mode       : $MODE"
 echo "Drive      : $DRIVE"
+echo "LiDAR      : ${LIDAR_VARIANT}°"
+echo "Dogs       : $ndog (axis-split=${NDOGS})"
 echo "Sheep      : $active active"
 echo "Policy dir : $RESOLVED_POLICY_DIR"
 echo "------------------------------------------------------------"
@@ -178,6 +229,7 @@ HERDING_POLICY_DIR=$RESOLVED_POLICY_DIR
 HERDING_DRIVE=$DRIVE
 HERDING_WORLD=$WORLD
 HERDING_LIDAR=$LIDAR_VARIANT
+HERDING_NDOGS=$NDOGS
 HERDING_USE_GT=${HERDING_USE_GT:-0}
 EOF