docs: unify language contract to Russian and reconcile the spec with the code

The docs folder had drifted into a mixed-language contract: IMPLEMENTATION_SPEC.md was
entirely in English while the rest was Russian with English headings. Prose is now
Russian throughout; English is kept only where it is technically load-bearing (commands,
tool output, config keys, file/module/table names, tc handles) and for terms with no
settled Russian equivalent in this context (drift, reconcile, peer, namespace, handshake).
docs/README.md now states that contract explicitly.

Rewriting the spec surfaced several places where it described intent rather than the
code. Corrected against traffic_core/src:

- module layout: the spec showed traffic/ and firewall/ as directories with
  model/inspect/apply submodules. They are single files, and sysctl_tuning.rs was
  missing from the tree entirely.
- data model: Peer was documented as carrying public key, handshake and counters. It
  carries only ipv4; runtime state lives in a separate PeerRuntime, and there is no
  desired-state struct at all - it is derived inline from config plus sorted peers.
  RuntimeTarget.netns_path is a String, not a PathBuf.
- interface detection: the spec listed a third step taking the first non-loopback
  interface with an IPv4 address. No such fallback exists; detection bails with an
  explicit error, which is safer and is now documented as deliberate.
- reconcile: the pseudo-flow folded firewall into reconcile_traffic. They are two
  independent reconciles with separate locks.
- locking: only traffic.lock was documented. There are three locks, and
  sysctl_tuning.lock has a 30s timeout rather than 10s. Stale-lock recovery via
  /proc/<pid> was undocumented.
- sysctl_tuning: the module also manages /etc/modules-load.d/90-awg2-harness-bbr.conf
  when BBR is active, and refuses to touch either managed file without its marker.
  Both docs and the acceptance checklist claimed only the sysctl.d file was written.
- status contract: the documented output was outdated. All 15 printed sections are now
  listed in the order status.rs emits them, including the degraded path when no container
  is found, and the fact that the Network tuning block is printed between the System
  service line and the container lines.
- swap: size was documented as a constant 2048 MB instead of swap.size_mb, and the
  /etc/fstab.awg2-harness.bak backup was undocumented.
- apply/verify: the tolerated-error lists are now the actual closed sets from the code.

Acceptance checklist gained checks for the daemon startup failure policy (sysctl/swap
fatal, traffic/firewall not), lock contention and stale-lock recovery, degraded status,
the modules-load file, and the marker guard. The stale host wg requirement was dropped
from its host-tools item.

Every path, table, counter and log string quoted in the docs was verified to exist in
the source. cargo test --workspace: 110 + 4 passed.

docs/CONFIGURATION.md

@@ -119,6 +119,9 @@ swappiness = 1
 `sysctl_tuning` — отдельный host-level модуль управляемого сетевого baseline через `sysctl`:
 
 - persistent-файл: `/etc/sysctl.d/90-awg2-harness-vpn.conf`;
+- дополнительный managed-файл `/etc/modules-load.d/90-awg2-harness-bbr.conf` создаётся только при фактически активном BBR и удаляется, когда BBR неактивен;
+- оба managed-файла несут в первой строке маркер `# Managed by awg2-harness sysctl_tuning`; файл без маркера не перезаписывается и не удаляется, вместо этого возвращается ошибка;
+- `net.ipv4.tcp_congestion_control` и `net.core.default_qdisc` управляются только при активном BBR;
 - runtime apply: только `sysctl -w` и только для drifted keys;
 - на установке `.deb` модуль не применяется;
 - на старте daemon модуль применяется, если `enabled=true` и `apply_on_daemon_start=true`;