fix(swap): bind ownership to device+inode, restrict swap.path to root-owned roots [P0]

Two defects that together made the previous ownership model unsafe, and reverse an earlier
deviation that the follow-up audit correctly rejected.

1. The sidecar marker identified a path, not a file.

   Ownership was `<swap.path>.awg2-harness-owned` sitting next to the swapfile, and the
   check was whether that sidecar existed and started with our marker. Nothing tied it to
   the file it was supposed to describe. The sequence that loses data: our swapfile is
   removed by hand, the sidecar survives, an operator puts an unrelated file at the same
   path - and the next reconcile calls it managed and, if the size differs, deletes it.
   The claim in the previous commit that a wrong path "can now only fail, never destroy"
   was wrong.

   Ownership now lives in /var/lib/awg2-harness/swap-ownership.conf, a root-only directory
   created 0700, and records version, path, device and inode. A path is not an identity: a
   file can be deleted and another created in its place, but (device, inode) cannot be
   reused while our file exists. Classification gained StaleOwnership for "right path,
   different file", which refuses exactly like Foreign but says which situation the
   operator is actually in. Only ManagedValid and ManagedWrongSize ever authorise a
   delete, and that is asserted directly.

   The record is written after the file exists, from its real metadata, so until it lands
   the swapfile is not considered managed. The old sidecar is now only ever removed, never
   read.

   classify_ownership is a pure function taking the recorded state and the observed
   identity, so the cases that matter - stale record, record for another path, no record -
   are unit-testable rather than requiring a Linux filesystem.

2. An arbitrary swap.path was still exploitable.

   The previous commit kept arbitrary absolute paths and argued the ownership marker was
   sufficient protection. It is not, and the audit is right to reject it. Between
   assert_swap_path_is_safe and fallocate/dd/mkswap running, a user who can write the
   parent directory replaces the final component with a symlink and redirects a root-owned
   operation. That is a TOCTOU; stat-ing before use cannot close it, because the check and
   the use are separate syscalls. Paths like /tmp/awg.swap or /home/user/awg.swap made this
   reachable by any local user.

   swap.path is now restricted to /swapfile or a file directly under
   /var/lib/awg2-harness/. Both are root-owned, so the window does not exist rather than
   being detected. /swapfile is kept so existing deployments need no config change. The
   deny-list it replaces only ruled out the paths someone had thought of; what actually
   matters is who can write the parent.

Eleven tests: the five classification states, a stale record, a record for another path,
the invariant that no refusal state is managed, record round-trip, malformed and
unsupported-version records, the newly allowed and newly rejected path shapes.

cargo test --workspace: 176 + 4 passed. clippy -D warnings clean.

docs/ACCEPTANCE_CHECKLIST.md

@@ -110,7 +110,10 @@
 - [ ] Если резервную копию записать не удалось, `/etc/fstab` остаётся неизменным.
 - [ ] `status` при нечитаемом `/etc/fstab` показывает ошибку, а не «нет swap-записей».
 - [ ] Путь swap с пробелом в имени попадает в `/etc/fstab` экранированным (`\040`) и корректно монтируется.
-- [ ] Рядом с управляемым swapfile создан маркер `<swap.path>.awg2-harness-owned`.
+- [ ] Создан `/var/lib/awg2-harness/swap-ownership.conf` с `device`, `inode` и путём; каталог `/var/lib/awg2-harness` имеет режим `0700` и принадлежит root.
+- [ ] Старый sidecar `<swap.path>.awg2-harness-owned` после обновления удалён и больше не создаётся.
+- [ ] Если swapfile удалить вручную и создать по тому же пути другой файл, `ensure-swap` завершается ошибкой `StaleOwnership` и **не** удаляет новый файл.
+- [ ] `swap.path` вне `/swapfile` и `/var/lib/awg2-harness/` отвергается на `validate-config` (проверить `/tmp/awg.swap`).
 - [ ] Если до запуска был активен другой swap, после `ensure-swap` он выключен.
 - [ ] После `ensure-swap` активен ровно один swap: `/swapfile`.
 - [ ] Все старые swap entries удалены из `/etc/fstab`.