Clarify connected_components candidate vertex semantics

[Copilot]

connected_components was easy to misread as a recursive graph traversal from the provided inputs. In practice, the input slice defines the candidate vertices whose neighbour sets are merged; neighbours discovered from those vertices are included in the result but are not recursively expanded unless they are also present in the input.

• API naming

  • Rename the input parameter from starts to candidates to reflect its actual role in the algorithm.

• Documentation

  • Update the API docs to state that only candidate vertices are queried via neighbours.
  • Explicitly call out that returned neighbours are grouped with their candidate vertex but are not explored further unless they also appear in candidates.

• Tests

  • Rename the relevant test and tighten its wording so the expected non-recursive behaviour is clear from the test itself.

let components = connected_components(&[0, 3], |node| match node {
    0 => [1].as_slice().iter().copied(),
    1 => [0, 2].as_slice().iter().copied(),
    2 => [1, 3].as_slice().iter().copied(),
    3 => [2].as_slice().iter().copied(),
    _ => unreachable!(),
});

In this shape, 0 and 3 are the candidate vertices. Their neighbour sets are merged into a single component, but 1 and 2 are not queried unless they are also included in the candidate list.

Close #840

[github-actions]

📊 iai-callgrind Benchmark Results

➡️ 20 Unchanged (within ±0.1%)

Click to expand

Benchmark	Instructions
iai_algos::corner_to_corner_astar	155,711
iai_algos::corner_to_corner_bfs	2,515,366
iai_algos::corner_to_corner_bfs_bidirectional	2,787,420
iai_algos::corner_to_corner_dfs	3,345,644
iai_algos::corner_to_corner_dijkstra	3,077,225
iai_algos::corner_to_corner_fringe	103
iai_algos::corner_to_corner_idastar	81,629
iai_algos::corner_to_corner_iddfs	3,712,137
iai_algos::no_path_astar	14
iai_algos::no_path_bfs	159
iai_algos::no_path_bfs_bidirectional	2,658,317
iai_algos::no_path_dfs	14
iai_algos::no_path_dijkstra	3,047,614
iai_algos::no_path_fringe	14
iai_edmondskarp::wikipedia_example_dense	22,382
iai_edmondskarp::wikipedia_example_sparse	46,083
iai_kuhn_munkres::kuhn_munkres_size_128	4,211,200
iai_kuhn_munkres::kuhn_munkres_size_32	239,050
iai_kuhn_munkres::kuhn_munkres_size_64	1,199,842
iai_separate_components::bench_separate_components	16,464,323

✅ All benchmarks unchanged

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iai-callgrind measures instructions executed, which is deterministic and not affected by system load.