refactor: one ownership model for every file written under /etc [P1]

Two modules wrote into /etc/sysctl.d and did it differently. sysctl_tuning refused to
touch a file whose first line lacked its marker and cleaned up on disable. swap wrote
/etc/sysctl.d/99-awg2-harness.conf with a different marker, no ownership check at all -
an administrator's file at that path was silently clobbered - and never removed it when
the module was turned off, so vm.swappiness stayed pinned forever.

New module managed_file.rs holds the rules once:

- a read that fails is never an absent file;
- ownership is Absent / Managed / Foreign, decided by the marker on the first line;
- write refuses Foreign, skips when the content already matches, and is otherwise atomic:
  temp sibling created with create_new, sync_all, mode 0644, rename, directory flush;
- remove refuses Foreign and treats Absent as success.

Both modules now go through it. sysctl_tuning loses its private write_atomic,
ensure_managed_or_absent and remove_managed_sysctl_file_at.

The swappiness file moves to /etc/sysctl.d/90-awg2-harness-swap.conf with the marker
"# Managed by awg2-harness swap", matching the 90- prefix every other managed file uses.
The old path is removed on first run of the new version, but only when it still carries
the previous "# Managed by awg2-harness" marker; a foreign file there is left alone and
does not fail the migration, since the current file lives elsewhere.

Disable semantics are now explicit: swap.enabled = false removes the managed file and
does nothing else. Runtime vm.swappiness is deliberately not reset - picking a value
would be inventing a policy - and active swap is not deactivated, because merely turning
a feature off should not trigger a destructive action. Previously ensure_swap returned
early and left the persisted file behind.

Also replaced a vacuous test: swappiness_conf_render_is_stable asserted that a string
literal equals a format! of the same literal, touching no production code at all. It now
exercises render_swappiness_conf, and a companion test proves the rendered file is
recognised as managed by the same marker check that guards overwriting it - otherwise
every reconcile would refuse to touch its own output.

Twelve tests: the three ownership states, refusal to overwrite and refusal to remove a
foreign file with content asserted intact, idempotent write, stale temp handling, an
unreadable path reported as an error, plus legacy migration in all three cases - ours,
foreign, absent.

cargo test --workspace: 148 + 4 passed. clippy -D warnings clean.

docs/ACCEPTANCE_CHECKLIST.md

@@ -129,6 +129,11 @@
 - [ ] Файл без marker по managed-пути не перезаписывается и не удаляется: команда завершается ошибкой.
 - [ ] В managed-файле отсутствует `udp_wmem_min`.
 - [ ] `net.ipv4.tcp_congestion_control` и `net.core.default_qdisc` присутствуют в managed-файле только при активном BBR.
+- [ ] `swap` пишет `vm.swappiness` в `/etc/sysctl.d/90-awg2-harness-swap.conf`, а не в `99-awg2-harness.conf`.
+- [ ] При обновлении с прежней версии `/etc/sysctl.d/99-awg2-harness.conf` удаляется, если несёт маркер `# Managed by awg2-harness`.
+- [ ] Файл по пути `99-awg2-harness.conf` без этого маркера остаётся нетронутым и не ломает `ensure-swap`.
+- [ ] При `swap.enabled = false` файл `90-awg2-harness-swap.conf` удаляется, активный swap **не** выключается, runtime `vm.swappiness` не переопределяется.
+- [ ] Любой managed-файл под `/etc`, у которого первая строка не содержит маркер владельца, не перезаписывается и не удаляется.
 - [ ] Runtime apply меняет только drifted keys (`sysctl -w`), без `sysctl --system`.
 - [ ] При `tcp_bbr = auto` и недоступном BBR сервис не падает, в статусе есть warning.
 - [ ] При `tcp_bbr = require` и недоступном BBR команда/старт завершается с ошибкой.