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>

tests/e2e/run.sh

@@ -21,7 +21,7 @@ case "${SWAPFORGE}" in
 esac
 echo "ok: resolved from the installed package, not the source tree"
 
-for binary in findmnt swapoff chmod mkswap swapon fallocate chattr btrfs dd; do
+for binary in findmnt swapoff fallocate chmod mkswap swapon chattr btrfs; do
     command -v "${binary}" >/dev/null || fail "${binary} is missing from the image"
 done
 echo "ok: every declared binary is present"

tests/unit/test_core.py

@@ -7,7 +7,6 @@ from swap_forge.core import (
     BTRFS_BINARIES,
     BTRFS_SWAP_PATH,
     DEFAULT_SWAP_PATH,
-    NON_BTRFS_BINARIES,
     REQUIRED_BINARIES,
     MissingBinaryError,
     create_swap,
@@ -22,28 +21,24 @@ from swap_forge.core import (
 
 class TestRequiredBinaries(unittest.TestCase):
     def test_declares_every_external_command_the_package_calls(self):
-        declared = (
-            set(REQUIRED_BINARIES) | set(BTRFS_BINARIES) | set(NON_BTRFS_BINARIES)
-        )
+        declared = set(REQUIRED_BINARIES) | set(BTRFS_BINARIES)
 
         self.assertEqual(
             declared,
             {
                 "findmnt",
                 "swapoff",
+                "fallocate",
                 "chmod",
                 "mkswap",
                 "swapon",
                 "chattr",
                 "btrfs",
-                "dd",
-                "fallocate",
             },
         )
 
     def test_filesystem_specific_binaries_are_not_declared_as_always_required(self):
         self.assertFalse(set(REQUIRED_BINARIES) & set(BTRFS_BINARIES))
-        self.assertFalse(set(REQUIRED_BINARIES) & set(NON_BTRFS_BINARIES))
 
     def test_missing_binary_is_reported_with_its_name(self):
         with mock.patch("swap_forge.core.subprocess.run") as subprocess_run:
@@ -153,7 +148,7 @@ class TestCreateSwap(unittest.TestCase):
             self.assertEqual(commands, ["fallocate", "chmod", "mkswap", "swapon"])
             self.assertEqual(fstab.read_text(), f"{swapfile} none swap sw 0 0\n")
 
-    def test_btrfs_disables_copy_on_write_and_uses_dd(self):
+    def test_btrfs_disables_copy_on_write_before_fallocate(self):
         with tempfile.TemporaryDirectory() as base:
             swapfile = Path(base) / "swap" / "swapfile"
             fstab = Path(base) / "fstab"
@@ -164,7 +159,11 @@ class TestCreateSwap(unittest.TestCase):
 
             commands = [call.args[0][0] for call in run.call_args_list]
             self.assertEqual(
-                commands, ["chattr", "btrfs", "dd", "chmod", "mkswap", "swapon"]
+                commands, ["chattr", "btrfs", "fallocate", "chmod", "mkswap", "swapon"]
+            )
+            self.assertEqual(
+                run.call_args_list[2].args[0],
+                ["fallocate", "-l", "1024M", str(swapfile)],
             )
             self.assertTrue(swapfile.parent.is_dir())