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/proxy/tests/masking_rng_hoist_perf_regression_tests.rs

@@ -0,0 +1,100 @@
+use super::*;
+use tokio::io::AsyncReadExt;
+use tokio::time::{Duration, timeout};
+
+async fn collect_padding(
+    total_sent: usize,
+    enabled: bool,
+    floor: usize,
+    cap: usize,
+    above_cap_blur: bool,
+    blur_max: usize,
+    aggressive: bool,
+) -> Vec<u8> {
+    let (mut tx, mut rx) = tokio::io::duplex(256 * 1024);
+
+    maybe_write_shape_padding(
+        &mut tx,
+        total_sent,
+        enabled,
+        floor,
+        cap,
+        above_cap_blur,
+        blur_max,
+        aggressive,
+    )
+    .await;
+
+    drop(tx);
+
+    let mut output = Vec::new();
+    timeout(Duration::from_secs(1), rx.read_to_end(&mut output))
+        .await
+        .expect("reading padded output timed out")
+        .expect("failed reading padded output");
+    output
+}
+
+#[tokio::test]
+async fn padding_output_is_not_all_zero() {
+    let output = collect_padding(1, true, 256, 4096, false, 0, false).await;
+
+    assert!(
+        output.len() >= 255,
+        "expected at least 255 padding bytes, got {}",
+        output.len()
+    );
+
+    let nonzero = output.iter().filter(|&&b| b != 0).count();
+    // In 255 bytes of uniform randomness, the expected number of zero bytes is ~1.
+    // A weak nonzero check can miss severe entropy collapse.
+    assert!(
+        nonzero >= 240,
+        "RNG output entropy collapsed, too many zero bytes: {} nonzero out of {}",
+        nonzero,
+        output.len(),
+    );
+}
+
+#[tokio::test]
+async fn padding_reaches_first_bucket_boundary() {
+    let output = collect_padding(1, true, 64, 4096, false, 0, false).await;
+    assert_eq!(output.len(), 63);
+}
+
+#[tokio::test]
+async fn disabled_padding_produces_no_output() {
+    let output = collect_padding(0, false, 256, 4096, false, 0, false).await;
+    assert!(output.is_empty());
+}
+
+#[tokio::test]
+async fn at_cap_without_blur_produces_no_output() {
+    let output = collect_padding(4096, true, 64, 4096, false, 0, false).await;
+    assert!(output.is_empty());
+}
+
+#[tokio::test]
+async fn above_cap_blur_is_positive_and_bounded_in_aggressive_mode() {
+    let output = collect_padding(4096, true, 64, 4096, true, 128, true).await;
+    assert!(!output.is_empty());
+    assert!(output.len() <= 128, "blur exceeded max: {}", output.len());
+}
+
+#[tokio::test]
+async fn stress_padding_runs_are_not_constant_pattern() {
+    // Stress and sanity-check: repeated runs should not collapse to identical
+    // first 16 bytes across all samples.
+    let mut first_chunks = Vec::new();
+    for _ in 0..64 {
+        let out = collect_padding(1, true, 64, 4096, false, 0, false).await;
+        first_chunks.push(out[..16].to_vec());
+    }
+
+    let first = &first_chunks[0];
+    let all_same = first_chunks.iter().all(|chunk| chunk == first);
+    assert!(
+        !all_same,
+        "all stress samples had identical prefix, rng output appears degenerate"
+    );
+}