fix(btrfs): allocate the swapfile with fallocate instead of dd

On a btrfs root the swapfile was written with dd from /dev/zero, so a
swapfile the size of the RAM cost tens of GiB of writes and stalled
every other writer on the filesystem. A caller that bounds the run, such
as an async Ansible task, killed dd halfway, and the next run found the
wrong size and started over. btrfs accepts a preallocated NOCOW
swapfile and the swap directory already carries +C, so every filesystem
now allocates with fallocate.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/swap_forge/core.py

@@ -10,9 +10,8 @@ DEFAULT_SWAP_PATH = Path("/swapfile")
 SIZE_PATTERN = re.compile(r"^(\d+)([gGmM]?[bB]?)?$")
 MIB_PER_GIB = 1024
 
-REQUIRED_BINARIES = ("findmnt", "swapoff", "chmod", "mkswap", "swapon")
-BTRFS_BINARIES = ("chattr", "btrfs", "dd")
-NON_BTRFS_BINARIES = ("fallocate",)
+REQUIRED_BINARIES = ("findmnt", "swapoff", "fallocate", "chmod", "mkswap", "swapon")
+BTRFS_BINARIES = ("chattr", "btrfs")
 EXIT_MISSING_BINARY = 127
 EXIT_COMMAND_FAILED = 1
 
@@ -117,8 +116,8 @@ def create_swap(path, size_mib, fs, fstab=FSTAB_FILE):
         fs: Filesystem type of the root mount.
         fstab: Path of the fstab file to append to.
     """
+    print(f"Creating {size_mib} MiB swapfile on {fs} at {path}")
     if fs == "btrfs":
-        print(f"Creating {size_mib} MiB swapfile on btrfs at {path}")
         path.parent.mkdir(parents=True, exist_ok=True)
         run(["chattr", "+C", str(path.parent)], check=False)
         run(
@@ -133,20 +132,8 @@ def create_swap(path, size_mib, fs, fstab=FSTAB_FILE):
             ],
             check=False,
         )
-        run(
-            [
-                "dd",
-                "if=/dev/zero",
-                f"of={path}",
-                "bs=1M",
-                f"count={size_mib}",
-                "status=progress",
-            ]
-        )
-    else:
-        print(f"Creating {size_mib} MiB swapfile on {fs} at {path}")
-        run(["fallocate", "-l", f"{size_mib}M", str(path)])
 
+    run(["fallocate", "-l", f"{size_mib}M", str(path)])
     run(["chmod", "600", str(path)])
     run(["mkswap", str(path)])
     run(["swapon", str(path)])