Security hardening, concurrency fixes, and expanded test coverage

This commit introduces a comprehensive set of improvements to enhance
the security, reliability, and configurability of the proxy server,
specifically targeting adversarial resilience and high-load concurrency.

Security & Cryptography:
- Zeroize MTProto cryptographic key material (`dec_key`, `enc_key`)
  immediately after use to prevent memory leakage on early returns.
- Move TLS handshake replay tracking after full policy/ALPN validation
  to prevent cache poisoning by unauthenticated probes.
- Add `proxy_protocol_trusted_cidrs` configuration to restrict PROXY
  protocol headers to trusted networks, rejecting spoofed IPs.

Adversarial Resilience & DoS Mitigation:
- Implement "Tiny Frame Debt" tracking in the middle-relay to prevent
  CPU exhaustion from malicious 0-byte or 1-byte frame floods.
- Add `mask_relay_max_bytes` to strictly bound unauthenticated fallback
  connections, preventing the proxy from being abused as an open relay.
- Add a 5ms prefetch window (`mask_classifier_prefetch_timeout_ms`) to
  correctly assemble and classify fragmented HTTP/1.1 and HTTP/2 probes
  (e.g., `PRI * HTTP/2.0`) before routing them to masking heuristics.
- Prevent recursive masking loops (FD exhaustion) by verifying the mask
  target is not the proxy's own listener via local interface enumeration.

Concurrency & Reliability:
- Eliminate executor waker storms during quota lock contention by replacing
  the spin-waker task with inline `Sleep` and exponential backoff.
- Roll back user quota reservations (`rollback_me2c_quota_reservation`)
  if a network write fails, preventing Head-of-Line (HoL) blocking from
  permanently burning data quotas.
- Recover gracefully from idle-registry `Mutex` poisoning instead of
  panicking, ensuring isolated thread failures do not break the proxy.
- Fix `auth_probe_scan_start_offset` modulo logic to ensure bounds safety.

Testing:
- Add extensive adversarial, timing, fuzzing, and invariant test suites
  for both the client and handshake modules.

src/config/types.rs

@@ -1450,6 +1450,14 @@ pub struct AntiCensorshipConfig {
     #[serde(default = "default_mask_shape_above_cap_blur_max_bytes")]
     pub mask_shape_above_cap_blur_max_bytes: usize,
 
+    /// Maximum bytes relayed per direction on unauthenticated masking fallback paths.
+    #[serde(default = "default_mask_relay_max_bytes")]
+    pub mask_relay_max_bytes: usize,
+
+    /// Prefetch timeout (ms) for extending fragmented masking classifier window.
+    #[serde(default = "default_mask_classifier_prefetch_timeout_ms")]
+    pub mask_classifier_prefetch_timeout_ms: u64,
+
     /// Enable outcome-time normalization envelope for masking fallback.
     #[serde(default = "default_mask_timing_normalization_enabled")]
     pub mask_timing_normalization_enabled: bool,
@@ -1488,6 +1496,8 @@ impl Default for AntiCensorshipConfig {
             mask_shape_bucket_cap_bytes: default_mask_shape_bucket_cap_bytes(),
             mask_shape_above_cap_blur: default_mask_shape_above_cap_blur(),
             mask_shape_above_cap_blur_max_bytes: default_mask_shape_above_cap_blur_max_bytes(),
+            mask_relay_max_bytes: default_mask_relay_max_bytes(),
+            mask_classifier_prefetch_timeout_ms: default_mask_classifier_prefetch_timeout_ms(),
             mask_timing_normalization_enabled: default_mask_timing_normalization_enabled(),
             mask_timing_normalization_floor_ms: default_mask_timing_normalization_floor_ms(),
             mask_timing_normalization_ceiling_ms: default_mask_timing_normalization_ceiling_ms(),