core: restore upstream shape for the shared mesh core

- Mesh, Dispatcher, Packet, packet pool and the core headers rebuilt from
  Arduino MeshCore's text; ZephCore changes re-applied, each fenced
  `// ZEPHCORE:` (diff vs upstream: 4540 -> 1147 lines)
- upstream header layout; MainBoard gains handleCommand/loop and the other
  upstream members; RNG::nextInt back in Utils.cpp
- packet logging moved to the roles' log hooks (helpers/PacketLog.h)
- zero-hop direct ACKs deduplicated and length-checked as upstream
- RadioIncludes.h -> src/, LoRaConfig.h -> helpers/
- .editorconfig: shared files keep upstream formatting

Verified: 10 role builds, per-function disassembly review, bench DM/ACK
regression (T1000-E, Wio L1, XIAO S3 on 1.17.4) incl. via live repeater.

zephcore/app/main_observer.cpp

@@ -35,7 +35,7 @@ LOG_MODULE_REGISTER(zephcore_observer_main, CONFIG_ZEPHCORE_MAIN_LOG_LEVEL);
 #include "../adapters/datastore/ZephyrFsFormat.h"
 #include <app/ObserverMesh.h>
 #include <adapters/clock/ZephyrRTCClock.h>
-#include <mesh/RadioIncludes.h>
+#include <src/RadioIncludes.h>
 #include <ZephyrWiFiStation.h>
 #include <ZephyrMQTTPublisher.h>
 #include "observer_creds.h"