bug(quake): validate_topology rejects connected manifests where bridges form a cycle (e.g. triangle A-B-C) #492

What

Subnets::validate_topology() (crates/quake/src/manifest/subnets.rs:148) rejects a connected
topology when subnets are bridged in a cycle. Three bridge nodes forming a triangle:

[nodes.bridge_bc]
subnets = ["B", "C"]

[nodes.bridge_ac]
subnets = ["A", "C"]

[nodes.bridge_ab]
subnets = ["A", "B"]

Every subnet is reachable from every other one, but Manifest::validate() (manifest.rs:1059) fails
with Destination subnet B not found; cannot merge subnets, so the manifest cannot be used at all.

Why

Walking the merge loop for the manifest above:

1. bridge_bc: C is merged into B; removed_subnets = {C → B}, working_map = {B, A}.
2. bridge_ac: C is already gone, so the else branch resolves C → B and A → A, moves A
   into B; removed_subnets = {C → B, A → B}, working_map = {B}.
3. bridge_ab: working_map.swap_remove("B") succeeds, because B is the surviving destination
   of the merged component, not a removed subnet. The code then computes
   destination = find_destination("A") = B, inserts B → B, and
   working_map.get_mut("B") is None (it was just removed), so it bails at line 211.

The swap_remove(subnet) fast path assumes that a subnet still present in working_map is in a
different component from first_subnet. That holds for chains and stars (the existing tests), but not
once a later bridge connects two subnets that are already in the same component through earlier
merges.

Repro

Adding this next to the existing tests in subnets.rs fails on main @ 6e76402:

#[test]
fn triangle_topology_is_connected() {
    let node_subnets: IndexMap<NodeName, Vec<SubnetName>> = [
        ("bridge_bc".to_string(), vec!["B".to_string(), "C".to_string()]),
        ("bridge_ac".to_string(), vec!["A".to_string(), "C".to_string()]),
        ("bridge_ab".to_string(), vec!["A".to_string(), "B".to_string()]),
    ]
    .into();

    assert!(Subnets::new(&node_subnets).validate_topology().is_ok());
}

cargo test -p quake --bins triangle_topology_is_connected → assertion failed: ...validate_topology().is_ok().

Suggested fix

Resolve both ends to their current representative before merging, and skip when they are equal,
i.e. a small union-find over subnet names instead of the swap_remove fast path plus the separate
"already merged" branch. Count the remaining roots at the end; more than one means the topology is
disconnected, same error as today. This also makes the result independent of node order. The
existing chain/star/disconnected tests stay as they are, plus the triangle test above.

Environment: main @ 6e76402.

(Opening as an issue first per CONTRIBUTING.md; happy to be assigned and submit the fix + test.)