Add support for --loopback-no-mix flag and topology synchronization

- Introduced the `--loopback-no-mix` flag to control 2-node vs 4-node loopback topologies.
- Updated topology handling logic to recompute and migrate as needed based on loopback flags.
- Enhanced database schema to include `loopback_no_mix` with automatic migration for backward compatibility.
- Refactored `create`, `edit`, and `route` commands to integrate new topology management capabilities.
- Improved `list` command output to include architecture details.

src/commands/create.rs

@@ -21,7 +21,7 @@ pub fn run(args: CreateArgs) -> Result<()> {
     let orig_sink = default_output("get-default-sink");
     let orig_source = default_output("get-default-source");
 
-    let stages = loopback::create_stages(&graph, &names)?;
+    let stage = loopback::create_simple_stage(&graph, &names, &[], &[])?;
 
     if let Some(sink) = &orig_sink {
         let _ = Command::new("pactl").args(["set-default-sink", sink]).status();
@@ -38,16 +38,19 @@ pub fn run(args: CreateArgs) -> Result<()> {
         ui::warn(&format!("removed {removed} unexpected monitor feedback link(s)."));
     }
 
+    // A fresh vmic always starts as Simple2Node - no source can be mixed in
+    // yet, so there's nothing for a 4-node split to isolate (see
+    // commands::topology). mid_name/mix_name are still stored even though no
+    // live node backs them yet: cheap, and needed if this vmic later
+    // upgrades. mix_node_id/mix_pid stay at their Default (None).
     let mut state = VmicState {
         name: name.clone(),
         sink_name: names.sink.clone(),
         mid_name: names.mid,
         mix_name: names.mix,
         source_name: names.source.clone(),
-        sink_node_id: Some(stages.sink_node_id),
-        mix_node_id: Some(stages.mix_node_id),
-        sink_pid: Some(stages.sink_pid as i64),
-        mix_pid: Some(stages.mix_pid as i64),
+        sink_node_id: Some(stage.sink_node_id),
+        sink_pid: Some(stage.pid as i64),
         ..Default::default()
     };