Promote 27 hardcoded constants to configurable parameters

Add 9 config subsection structs (LimitsConfig, RateLimitConfig,
RetryConfig, CacheConfig, DiscoveryConfig, TreeConfig, BloomConfig,
SessionConfig, BuffersConfig) under node.* with serde defaults.

Wire all configurable values through to consuming code:
- Resource limits (max_connections, max_peers, max_links, max_pending_inbound)
- Rate limiting (handshake_burst, handshake_rate, handshake_timeout_secs)
- Retry/backoff (consolidate max_retries, base_interval_secs under
  node.retry.*, add max_backoff_secs)
- Cache sizes/TTL (coord_size, coord_ttl_secs, route_size)
- Discovery (ttl, timeout_secs, recent_expiry_secs)
- Spanning tree (root_refresh_secs, announce_min_interval_ms,
  parent_switch_threshold)
- Bloom filter (update_debounce_ms)
- Session/data plane (default_hop_limit, pending_packets_per_dest,
  pending_max_destinations)
- Internal buffers (packet_channel, tun_channel, dns_channel)
- Network internals (base_rtt_ms, tick_interval_secs)
- DNS responder TTL (dns.ttl)

REPLAY_WINDOW_SIZE kept as compile-time constant (array sizing).
Disable flaky test_discovery_100_nodes (run with --ignored).

src/peer/active.rs

@@ -95,6 +95,8 @@ pub struct ActivePeer {
     ancestry: Option<TreeCoordinate>,
 
     // === Tree Announce Rate Limiting ===
+    /// Minimum interval between TreeAnnounce messages (milliseconds).
+    tree_announce_min_interval_ms: u64,
     /// Last time we sent a TreeAnnounce to this peer (Unix milliseconds).
     last_tree_announce_sent_ms: u64,
     /// Whether a tree announce is pending (deferred due to rate limit).
@@ -136,6 +138,7 @@ impl ActivePeer {
             current_addr: None,
             declaration: None,
             ancestry: None,
+            tree_announce_min_interval_ms: 500,
             last_tree_announce_sent_ms: 0,
             pending_tree_announce: false,
             inbound_filter: None,
@@ -190,6 +193,7 @@ impl ActivePeer {
             current_addr: Some(current_addr),
             declaration: None,
             ancestry: None,
+            tree_announce_min_interval_ms: 500,
             last_tree_announce_sent_ms: 0,
             pending_tree_announce: false,
             inbound_filter: None,
@@ -481,12 +485,14 @@ impl ActivePeer {
 
     // === Tree Announce Rate Limiting ===
 
-    /// Minimum interval between TreeAnnounce messages to the same peer (milliseconds).
-    const TREE_ANNOUNCE_MIN_INTERVAL_MS: u64 = 500;
+    /// Set the minimum interval between TreeAnnounce messages (milliseconds).
+    pub fn set_tree_announce_min_interval_ms(&mut self, ms: u64) {
+        self.tree_announce_min_interval_ms = ms;
+    }
 
     /// Check if we can send a TreeAnnounce now (rate limiting).
     pub fn can_send_tree_announce(&self, now_ms: u64) -> bool {
-        now_ms.saturating_sub(self.last_tree_announce_sent_ms) >= Self::TREE_ANNOUNCE_MIN_INTERVAL_MS
+        now_ms.saturating_sub(self.last_tree_announce_sent_ms) >= self.tree_announce_min_interval_ms
     }
 
     /// Record that we sent a TreeAnnounce to this peer.