Add RRT online replanning in continuous space (navigation/41)

[rsasaki0109]

What

A new example, examples/navigation/41_rrt_replanning.py: a Rapidly-exploring Random Tree (LaValle, 1998) running the online-replanning loop in continuous space. The repo had grid A* online replanning (04_online_replanning_astar.py) but no sampling-based planner — this adds the continuous-space counterpart.

The lesson

RRT plans by growing a tree from the start (sample a point, extend the nearest node a short step toward it, keep collision-free edges) until it reaches the goal. The robot plans with the obstacles it knows and follows the path, but one obstacle is hidden until it comes within sensor range. When a newly sensed obstacle is found to block the remaining path, the world reports path_blocked, the agent replans an RRT from its current position, and it continues. Plan, discover, replan, arrive.

RRT is randomized, so the first tree occasionally routes around the hidden disc and no replan is needed — the robot reaches the goal either way; the default seed always replans.

Contents

• A self-contained RRTWorld (known + hidden discs, local sensing, segment–disc collision), a standalone plan_rrt() (nearest-node extension, goal bias, reconstruction), and an RRTReplanAgent that detects a blocked remaining path and replans.
• Two smoke tests: the default seed threads the gap, senses the disc, replans, and reaches the goal with no collision; the planner + replanner reach the goal on every seed in range(6).
• examples/README.md row + a full examples/navigation/README.md section; example count 43→44 and test count 120→122 in README.md / docs/status.md.

Verification

• Seeds 0–9: 10/10 reach the goal, ~8/10 trigger a replan; the default seed replans once.
• Full suite green (135 passed); matplotlib render path confirmed under Agg.

References

• S. M. LaValle, "Rapidly-Exploring Random Trees: A New Tool for Path Planning," TR 98-11, Iowa State University, 1998.
• S. M. LaValle & J. J. Kuffner, "Randomized Kinodynamic Planning," IJRR 2001.

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