rekey: apply symmetric jitter to desynchronize dual-initiation

Add a per-session signed jitter offset (uniform [-15, +15] seconds)
to the rekey timer triggers in check_rekey (FMP) and check_session_rekey
(FSP). The configured `node.rekey.after_secs` becomes the nominal
interval rather than a floor; mean is preserved. Desynchronizes
both endpoints in symmetric-start meshes so the dual-initiation
race stops occurring rather than being resolved after the fact by
the smaller-NodeAddr tie-breaker.

Per-session storage means each rekey cutover reconstructs the
session and redraws the jitter naturally — successive cycles get
independent offsets, preventing drift back into sync.

src/node/mod.rs

@@ -24,6 +24,13 @@ pub(crate) mod wire;
 use self::discovery_rate_limit::{DiscoveryBackoff, DiscoveryForwardRateLimiter};
 use self::rate_limit::HandshakeRateLimiter;
 use self::routing_error_rate_limit::RoutingErrorRateLimiter;
+
+/// Half-range of the symmetric jitter applied to the per-session rekey timer.
+/// Each session draws an offset uniformly from `[-REKEY_JITTER_SECS,
+/// +REKEY_JITTER_SECS]` seconds at construction. Desynchronizes
+/// dual-initiation in symmetric-start meshes; the configured
+/// `node.rekey.after_secs` remains the nominal interval (mean preserved).
+pub(crate) const REKEY_JITTER_SECS: i64 = 15;
 use self::wire::{
     FLAG_CE, FLAG_KEY_EPOCH, FLAG_SP, build_encrypted, build_established_header,
     prepend_inner_header,