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.

CHANGELOG.md

@@ -25,6 +25,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Apply ±15s symmetric jitter per session to the FMP and FSP rekey
+  timer trigger. Eliminates the steady-state dual-initiation race
+  in symmetric-start meshes; previously the smaller-NodeAddr
+  tie-breaker resolved correctness after every cycle's collision.
+  `node.rekey.after_secs` becomes the nominal interval rather than
+  a floor; mean is preserved.
 - Rekey integration test (`testing/static/scripts/rekey-test.sh`):
   bumped Phase 1 baseline-convergence headroom from 36s to 60s.
   Eliminates the intermittent GitHub-runner Phase 1 timeout that