Pin consecutive decrypt-failure counter and threshold-20 force removal

Cover the security-relevant defensive signal: sustained decrypt
failures indicate key drift or active probing; threshold-trip force-
removes the peer.

src/peer/active.rs (+43): two unit tests on the counter struct itself
- test_increment_decrypt_failures_monotonic asserts each
  increment_decrypt_failures() call returns count+1 for at least 25
  iterations
- test_reset_decrypt_failures_zeroes_counter asserts the reset
  helper zeroes a non-zero counter and is idempotent

src/node/tests/decrypt_failure.rs (new, 93 lines): end-to-end test
- Builds a Node + connected peer via existing make_completed_connection
  / add_connection / promote_connection harness so peers_by_index is
  exercised, not just peers
- Drives the peer to threshold-20 by calling handle_decrypt_failure 20
  times; asserts iterations 1..20 leave the peer registered with
  monotonically increasing counter, then iteration 20 evicts from both
  peers and peers_by_index

src/node/handlers/encrypted.rs: visibility-only widen on
handle_decrypt_failure from private to pub(in crate::node) so the
in-tree test can drive the threshold logic without re-implementing
it. Same pattern as the already-pub(in crate::node)
handle_encrypted_frame in the same file.

Threshold pinned: DECRYPT_FAILURE_THRESHOLD = 20 at
src/node/handlers/encrypted.rs:11.

src/node/tests/decrypt_failure.rs

@@ -0,0 +1,93 @@
+//! Tests for the consecutive-decrypt-failure threshold force-removal path.
+//!
+//! Covers `Node::handle_decrypt_failure` (in `node/handlers/encrypted.rs`),
+//! which increments `ActivePeer::increment_decrypt_failures` on each AEAD
+//! verification failure and force-removes the peer once
+//! `DECRYPT_FAILURE_THRESHOLD` consecutive failures are observed. The
+//! threshold is a defensive signal against a peer whose session is
+//! desynchronized or under attack, so regression coverage of the wiring
+//! between counter, threshold, and peer eviction is security-relevant.
+
+use super::*;
+
+/// Drive a fully-promoted peer to the decrypt-failure threshold and verify
+/// it is removed from both `peers` and `peers_by_index`.
+///
+/// Setup uses the `make_completed_connection` harness so the peer has a
+/// real `our_index`/`transport_id`, ensuring `remove_active_peer` exercises
+/// the full `peers_by_index` cleanup path (not just the bare `peers` table).
+#[test]
+fn test_decrypt_failure_threshold_removes_peer() {
+    // Threshold constant in node/handlers/encrypted.rs (kept in sync with
+    // production code; see DECRYPT_FAILURE_THRESHOLD).
+    const THRESHOLD: u32 = 20;
+
+    let mut node = make_node();
+    let transport_id = TransportId::new(1);
+    let link_id = LinkId::new(1);
+
+    // Build a fully-promoted active peer with our_index/transport_id set
+    // so peers_by_index is populated by promote_connection.
+    let (conn, identity) = make_completed_connection(&mut node, link_id, transport_id, 1_000);
+    let node_addr = *identity.node_addr();
+
+    node.add_connection(conn).unwrap();
+    node.promote_connection(link_id, identity, 2_000).unwrap();
+
+    // Sanity: peer is registered and indexed.
+    assert_eq!(node.peer_count(), 1, "peer should be present after promote");
+    let our_index = node
+        .get_peer(&node_addr)
+        .and_then(|p| p.our_index())
+        .expect("promoted peer must have our_index");
+    assert_eq!(
+        node.peers_by_index.get(&(transport_id, our_index.as_u32())),
+        Some(&node_addr),
+        "peers_by_index must be populated after promote"
+    );
+    assert_eq!(
+        node.get_peer(&node_addr)
+            .unwrap()
+            .consecutive_decrypt_failures(),
+        0,
+        "fresh peer's failure counter must start at zero"
+    );
+
+    // Drive failures up to (but not including) the threshold; peer must
+    // remain present and the counter must increase monotonically.
+    for expected in 1..THRESHOLD {
+        node.handle_decrypt_failure(&node_addr);
+        let count = node
+            .get_peer(&node_addr)
+            .expect("peer must still be present below threshold")
+            .consecutive_decrypt_failures();
+        assert_eq!(
+            count, expected,
+            "counter should track failures pre-threshold"
+        );
+    }
+    assert_eq!(
+        node.peer_count(),
+        1,
+        "peer must remain registered until threshold is reached"
+    );
+
+    // The Nth failure crosses the threshold and triggers force-removal.
+    node.handle_decrypt_failure(&node_addr);
+
+    assert!(
+        node.get_peer(&node_addr).is_none(),
+        "peer must be removed from peers table at threshold"
+    );
+    assert_eq!(
+        node.peer_count(),
+        0,
+        "peer_count must be zero after eviction"
+    );
+    assert!(
+        !node
+            .peers_by_index
+            .contains_key(&(transport_id, our_index.as_u32())),
+        "peers_by_index entry must be cleaned up at threshold"
+    );
+}

src/node/tests/mod.rs

@@ -10,6 +10,7 @@ mod ble;
 mod bloom;
 mod bloom_poison;
 mod bootstrap;
+mod decrypt_failure;
 mod disconnect;
 mod discovery;
 #[cfg(target_os = "linux")]