feat: add astar_reach for steppable A* search

[tachsin]

Summary

Implements astar_reach for #775.

A* currently runs to completion. There is no way to step it, inspect expansions, or stop early. dijkstra_reach, bfs_reach, and dfs_reach already provide that for the other searches.

astar_reach(start, successors, heuristic) yields one AstarReachableItem per expansion:

• node, parent, total_cost (g) — same idea as DijkstraReachableItem
• estimated_cost (f = g + h) — so a visualizer can show the A* priority

Nodes come out in the same order astar would expand them. Each node is yielded at most once. There is no goal predicate; stop from the outside with find, take_while, or by dropping the iterator.

idastar / iddfs are not in this PR. They restart the search at each bound, so a useful iterator is a different design.

Closes#775

Test plan

• cargo test --test astar-reach
• Zero heuristic matches dijkstra_reach on the same graph
• Stopping at the goal reports the same cost as astar
• A consistent heuristic expands fewer nodes than h = 0

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