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/load.rs

@@ -430,6 +430,25 @@ impl ProxyConfig {
             ));
         }
 
+        if config.censorship.mask_relay_max_bytes == 0 {
+            return Err(ProxyError::Config(
+                "censorship.mask_relay_max_bytes must be > 0".to_string(),
+            ));
+        }
+
+        if config.censorship.mask_relay_max_bytes > 67_108_864 {
+            return Err(ProxyError::Config(
+                "censorship.mask_relay_max_bytes must be <= 67108864".to_string(),
+            ));
+        }
+
+        if !(5..=50).contains(&config.censorship.mask_classifier_prefetch_timeout_ms) {
+            return Err(ProxyError::Config(
+                "censorship.mask_classifier_prefetch_timeout_ms must be within [5, 50]"
+                    .to_string(),
+            ));
+        }
+
         if config.censorship.mask_timing_normalization_ceiling_ms
             < config.censorship.mask_timing_normalization_floor_ms
         {
@@ -1134,6 +1153,10 @@ mod load_security_tests;
 #[path = "tests/load_mask_shape_security_tests.rs"]
 mod load_mask_shape_security_tests;
 
+#[cfg(test)]
+#[path = "tests/load_mask_classifier_prefetch_timeout_security_tests.rs"]
+mod load_mask_classifier_prefetch_timeout_security_tests;
+
 #[cfg(test)]
 mod tests {
     use super::*;