Fix remove_cycles_dfs missing cycles due to wrong back-edge parent

[Copilot]

remove_cycles would silently leave cycles intact after crossover. The colored DFS used prev: Option<NeuronLocation> to identify the back-edge, but HashMap's non-deterministic iteration order could cause prev to hold a grandparent (or earlier ancestor) rather than the true immediate parent. The guard self[prev].outputs.contains_key(&current) would then return false, and the cycle edge was never added to edges_to_remove.

Changes

• remove_cycles_dfs: Replace prev: Option<NeuronLocation> parameter with an explicit path: &mut Vec<NeuronLocation> DFS stack. path.last() is guaranteed to be the immediate predecessor of current at the point a back-edge is detected, regardless of HashMap iteration order.

// before – prev could be a wrong ancestor
fn remove_cycles_dfs(..., prev: Option<NeuronLocation>, current: NeuronLocation) {
    if existing == 0 {
        let prev = prev.unwrap();
        if self[prev].outputs.contains_key(&current) {  // silently skipped when prev was wrong
            edges_to_remove.insert(Connection { from: prev, to: current });
        }
    }
}

// after – path.last() is always the true calling node
fn remove_cycles_dfs(..., path: &mut Vec<NeuronLocation>, current: NeuronLocation) {
    if existing == 0 {
        if let Some(&parent) = path.last() {
            edges_to_remove.insert(Connection { from: parent, to: current });
        }
    }
    visited.insert(current, 0);
    path.push(current);
    // ... recurse ...
    path.pop();
    visited.insert(current, 1);
}

• Cleanup: Remove all temporary debug assertions added during investigation from split_connection, add_connection, and crossover.

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[HyperCodec]

LGTM. idk if I really care about having all these extra debug test cases, but it's probably better to have them than not.