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

src/types.rs

@@ -29,11 +29,15 @@ pub struct DeviceSelection {
 #[serde(rename_all = "snake_case")]
 pub enum Topology {
     /// Single eligible disk; btrfs on remainder.
-    Single,
+    BtrfsSingle,
+    /// Single eligible disk; bcachefs on remainder.
+    BcachefsSingle,
     /// Two eligible disks; independent btrfs on each data partition.
     DualIndependent,
     /// SSD + HDD; bcachefs with SSD cache/promote and HDD backing.
     SsdHddBcachefs,
+    /// Two-disk bcachefs layout using both data partitions (2 copies semantics).
+    Bcachefs2Copy,
     /// Optional mirrored btrfs across two disks when explicitly requested.
     BtrfsRaid1,
 }