Optimize midpoint computation in island detection BFS

[Copilot]

Addresses feedback to eliminate redundant computation in the sequential island detection algorithm. The start edge midpoint was being recomputed for every edge visited during BFS traversal, despite remaining constant throughout each component.

Changes:

• Compute start_midpoint once when initializing each component (before BFS loop)
• Pass pre-computed value to is_component_island_sequential instead of recomputing on every iteration

Before:

loop {
    // ... inside BFS loop
    let current_distance = is_component_island_sequential(
        &current_edge,
        compute_midpoint(start_edge, vertices),  // ❌ Recomputed every iteration
        // ...
    );
}

After:

let start_midpoint = compute_midpoint(start_edge, vertices);  // ✓ Computed once

loop {
    // ... inside BFS loop
    let current_distance = is_component_island_sequential(
        &current_edge,
        start_midpoint,  // ✓ Reused
        // ...
    );
}

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