feat: add BMSSP single-source shortest paths

[tachsin]

Summary

Implements the SSSP algorithm from Duan, Mao, Mao, Shu, Yin 2025 for #730.

The paper's BMSSP recursion (FindPivots, base-case Dijkstra, bounded multi-source calls) is wired to the same successor-function model as dijkstra / dijkstra_all:

• sssp_all(start, successors) -> HashMap<N, (N, C)>
• sssp(start, successors, success) -> Option<(Vec<N>, C)>

build_path works on the sssp_all map.

What this is not

• It does not claim the paper's (O(m\log^{2/3}n)) bound. The Lemma 3.3 block queue is a BTreeMap, and the reachable implicit graph is materialized first.
• sssp computes all distances, then picks a cheapest successful node. Use dijkstra when the successor graph is unbounded or you only need one target.

A linear repair pass at the end fixes nodes left stale when many paths share a length. The paper assumes unique path lengths; unit-cost grids do not.

Test plan

• cargo test --test sssp
• Costs match dijkstra_all on the small tree, random graphs, and a grid
• sssp path to a goal matches dijkstra on a finite graph

Closes#730

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