MPPI planner + fence-line localization + scan obstacles

- Lightweight MPPI (200 trajectories, 15 steps, endpoint path_align)
- adaptive exploration (more random when stuck)
- fence-line Hough detection + geometric correction
- scan hit points marked at 255 immediately
- forward half-step pre-check for obstacle avoidance
- Ackermann constraint |wz| <= |vx|/min_turn_r
- XORshift RNG (stdlib broken on ARM)

config/params.yaml

@@ -0,0 +1,4 @@
+nav_lite_node:
+  ros__parameters:
+    map_file: ""          # PGM path, set via launch argument
+    lookahead: 0.5        # Pure Pursuit lookahead distance (m)