fix: stabilize tunnel downloads (deadlock, stuck-consumer eviction, real diagnostics)

Downloads were dying and stalling under real network conditions. Root causes
fixed in the muxer's stream-delivery path:

- dispatch_to_local held a DashMap Ref across a nested remove_stream call,
  self-deadlocking the leg's reader task whenever a stream's backlog needed
  eviction — permanently losing a tokio worker thread per occurrence.
- Eviction now runs off the hot path entirely, in a periodic background
  reaper, and only fires when a stream is both over its byte budget AND has
  made no delivery progress for an RTT-adaptive grace window (was a flat
  300ms then 5s, which killed merely-slow-but-alive consumers).
- The server never told the client when a bridge ended normally (only on
  failed connect), leaving the client's virtual TCP socket stuck in
  CloseWait for a full idle timeout and slowing the whole engine loop.
- The client's upload path silently dropped in-flight chunks when all tunnel
  legs were briefly down instead of pausing and retrying like the server
  bridge already did.
- Tried end-to-end credit-based flow control for the download producer;
  reverted the enforcement (kept the dormant frame/API) after it proved
  worse than the pre-existing local channel backpressure — RTT-coupled
  pauses produced burst-then-stall downloads and jitter/ping spikes on the
  shared physical leg.
- Server-side diagnostics snapshot was a stub (always-empty tunnel state,
  misleading placeholder trigger); now aggregates real per-session Muxer
  metrics from the SessionManager.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

core/src/nrxp/frame.rs

@@ -35,6 +35,10 @@ pub(crate) enum FrameType {
     /// путь: едет по контрольному каналу, сервер сохраняет его в пер-сессионный
     /// файл. Никогда не маршрутизируется в локальные сокеты.
     Diag = 0x06,
+    /// Кредит потока для сквозного flow-control (payload — `u32` BE, "можешь
+    /// прислать ещё N байт"). Отправляется приёмной стороной по мере
+    /// освобождения локального буфера — см. `Muxer::grant_credit`/`consume_credit`.
+    Credit = 0x07,
 }
 
 /// Разобранный заголовок кадра (25 байт). Поля идут в том же порядке, что и в wire.
@@ -170,6 +174,7 @@ impl Parser for FrameHeader {
             0x04 => FrameType::UdpConnect,
             0x05 => FrameType::UdpData,
             0x06 => FrameType::Diag,
+            0x07 => FrameType::Credit,
             unknown => {
                 // After successful AEAD decryption an unknown frame type means a
                 // protocol version mismatch or data corruption that the cipher