acw: airtime-scale jitter caps, companion surroundings awareness

- flood retransmit jitter now capped at min(2000ms, 6·airtime) instead of
  fixed 2000ms — spreads tighter at SF7, unchanged at SF8
- reactive per-dupe backoff cap now min(2000ms, 12·airtime), keeps
  semantic of "push past ~12 relay slots"
- contention ring 16 → 24 for 50-neighbor hilltops
- companions passively track heard floods (warms EMA without forwarding)
  and spread their own TX by up to min(1000ms, 3·airtime), hopefully
  fixing repeaters missing companion's first transmission

config cleanup:
- move BLE TX buffer bumps (ACL_TX=12 etc.) from zephcore_common.conf to
  esp32_common.conf — the Espressif blob needs them, nRF doesn't, and
  the bumps were overflowing nRF52840 RAM
- remove CONFIG_ZEPHCORE_MAX_CONTACTS=510 overrides from 5 nRF52840
  companion boards; Kconfig default of 350 fits with comfortable margin
  (wio prod: 91% → 79% RAM)

zephcore/app/RepeaterMesh.cpp

@@ -522,10 +522,14 @@ void RepeaterMesh::logTxFail(mesh::Packet* pkt, int len) {
 
 uint32_t RepeaterMesh::getRetransmitDelay(const mesh::Packet* packet) {
     float factor = getContentionTracker().getFloodDelayFactor();
-    uint32_t t = (uint32_t)(_radio->getEstAirtimeFor(
-        packet->getPathByteLen() + packet->payload_len + 2) * factor);
-    uint32_t max_jitter = 5 * t;
-    /* Cap jitter to 2000ms to avoid excessive latency in very dense areas.
+    uint32_t airtime = _radio->getEstAirtimeFor(
+        packet->getPathByteLen() + packet->payload_len + 2);
+    uint32_t max_jitter = (uint32_t)(5 * airtime * factor);
+    /* Airtime-scaled ceiling: never exceed ~6 airtimes of spread
+     * (keeps SF7/narrow-BW configs from wasting time in oversized jitter windows). */
+    uint32_t airtime_cap = 6 * airtime;
+    if (max_jitter > airtime_cap) max_jitter = airtime_cap;
+    /* Absolute cap: avoid excessive latency in very dense areas.
      * Reactive backoff will fine-tune further if needed. */
     if (max_jitter > 2000) max_jitter = 2000;
     /* Floor: give downstream nodes time to finish RX processing