topology: add bcachefs-2copy; add bcachefs-single; rename single->btrfs-single; update planner, fs mapping, CLI, defaults, preview topo strings, README

src/config/loader.rs

@@ -187,9 +187,11 @@ pub fn validate(cfg: &Config) -> Result<()> {
 
     // Topology-specific quick checks (basic for now)
     match cfg.topology {
-        Topology::Single => {} // nothing special
+        Topology::BtrfsSingle => {} // nothing special
+        Topology::BcachefsSingle => {}
         Topology::DualIndependent => {}
         Topology::SsdHddBcachefs => {}
+        Topology::Bcachefs2Copy => {}
         Topology::BtrfsRaid1 => {
             // No enforced requirement here beyond presence of two disks at runtime.
             if cfg.filesystem.btrfs.raid_profile != "raid1" && cfg.filesystem.btrfs.raid_profile != "none" {
@@ -352,7 +354,7 @@ fn default_config() -> Config {
             allow_removable: false,
             min_size_gib: 10,
         },
-        topology: Topology::Single,
+        topology: Topology::BtrfsSingle,
         partitioning: Partitioning {
             alignment_mib: 1,
             require_empty_disks: true,