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/state.rs

@@ -29,6 +29,11 @@ pub struct VmicState {
     // pitfalls here, unlike PipeWire's JSON dump (see pw::text).
     pub mic_source: Option<String>,
     pub mic_volume_pct: Option<u8>,
+
+    // Opt-in: keep the 2-node topology even when a source is mixed in while
+    // the self-monitor loopback is on, accepting that the mix becomes
+    // audible in the loopback instead of paying for a 4-node split.
+    pub loopback_no_mix: bool,
 }
 
 fn db_path() -> PathBuf {
@@ -90,12 +95,33 @@ pub fn open_db() -> Result<Connection> {
             loopback_id     INTEGER,
             volume_pct      INTEGER,
             mic_source      TEXT,
-            mic_volume_pct  INTEGER
+            mic_volume_pct  INTEGER,
+            loopback_no_mix INTEGER NOT NULL DEFAULT 0
         );",
     )?;
+    migrate_add_loopback_no_mix(&conn)?;
     Ok(conn)
 }
 
+/// `CREATE TABLE IF NOT EXISTS` above only covers fresh databases; an
+/// existing `vmic.db` from before `loopback_no_mix` existed needs the column
+/// added explicitly. Safe to call unconditionally - a no-op once the column
+/// is present either way.
+fn migrate_add_loopback_no_mix(conn: &Connection) -> Result<()> {
+    let has_column = conn
+        .prepare("PRAGMA table_info(vmics)")?
+        .query_map([], |row| row.get::<_, String>(1))?
+        .filter_map(std::result::Result::ok)
+        .any(|c| c == "loopback_no_mix");
+    if !has_column {
+        conn.execute(
+            "ALTER TABLE vmics ADD COLUMN loopback_no_mix INTEGER NOT NULL DEFAULT 0",
+            [],
+        )?;
+    }
+    Ok(())
+}
+
 impl VmicState {
     fn from_row(row: &rusqlite::Row) -> rusqlite::Result<Self> {
         Ok(Self {
@@ -112,6 +138,7 @@ impl VmicState {
             volume_pct: row.get("volume_pct")?,
             mic_source: row.get("mic_source")?,
             mic_volume_pct: row.get("mic_volume_pct")?,
+            loopback_no_mix: row.get("loopback_no_mix")?,
         })
     }
 
@@ -135,8 +162,8 @@ impl VmicState {
             "INSERT INTO vmics (
                 name, sink_name, mid_name, mix_name, source_name,
                 sink_node_id, mix_node_id, sink_pid, mix_pid, loopback_id, volume_pct,
-                mic_source, mic_volume_pct
-            ) VALUES (?1, ?2, ?3, ?4, ?5, ?6, ?7, ?8, ?9, ?10, ?11, ?12, ?13)
+                mic_source, mic_volume_pct, loopback_no_mix
+            ) VALUES (?1, ?2, ?3, ?4, ?5, ?6, ?7, ?8, ?9, ?10, ?11, ?12, ?13, ?14)
             ON CONFLICT(name) DO UPDATE SET
                 sink_name = excluded.sink_name,
                 mid_name = excluded.mid_name,
@@ -149,7 +176,8 @@ impl VmicState {
                 loopback_id = excluded.loopback_id,
                 volume_pct = excluded.volume_pct,
                 mic_source = excluded.mic_source,
-                mic_volume_pct = excluded.mic_volume_pct",
+                mic_volume_pct = excluded.mic_volume_pct,
+                loopback_no_mix = excluded.loopback_no_mix",
             params![
                 self.name,
                 self.sink_name,
@@ -164,6 +192,7 @@ impl VmicState {
                 self.volume_pct,
                 self.mic_source,
                 self.mic_volume_pct,
+                self.loopback_no_mix,
             ],
         )?;
         Ok(())