Deprecate/block packets with a missing/invalid hop_start value (pre-hop firmware) (related to issue #7369) (#9476)

* Deprecate forwarding for invalid hop_start

* Add pre-hop packet drop policy

* Log ignored rebroadcasts for pre-hop packets

* Respect pre-hop policy ALLOW in routing gates

* Exempt local packets from pre-hop drop policy

* Format pre-hop log line

* Add MODERN_ONLY rebroadcast mode for pre-hop packets

* Simplify implementation for drop packet only behaviour

* Revert formatting-only changes

* Match ReliableRouter EOF formatting

* Make pre-hop drop a build-time flag

* Rework to compile/build flag MESHTASTIC_PREHOP_DROP

* Set MESHTASTIC_PREHOP_DROP off by default

* Inline pre-hop hop_start validity check

---------

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>
Co-authored-by: Jord <650645+DivineOmega@users.noreply.github.com>

src/mesh/NodeDB.h

@@ -114,6 +114,27 @@ uint32_t sinceReceived(const meshtastic_MeshPacket *p);
 /// Returns defaultIfUnknown if the number of hops couldn't be determined.
 int8_t getHopsAway(const meshtastic_MeshPacket &p, int8_t defaultIfUnknown = -1);
 
+enum class HopStartStatus : uint8_t { VALID = 0, MISSING_OR_UNKNOWN, INVALID };
+
+/// Classify hop_start validity for forwarding decisions.
+HopStartStatus classifyHopStart(const meshtastic_MeshPacket &p);
+
+inline bool shouldDropPacketForPreHop(const meshtastic_MeshPacket &p)
+{
+#if !MESHTASTIC_PREHOP_DROP
+    (void)p;
+    return false;
+#else
+    if (isFromUs(&p)) {
+        return false; // local-originated packets should never be dropped by pre-hop drop policy
+    }
+    return classifyHopStart(p) != HopStartStatus::VALID;
+#endif
+}
+
+/// Rate-limited debug log when hop_start is invalid/missing and packet is dropped.
+void logHopStartDrop(const meshtastic_MeshPacket &p, const char *context);
+
 enum LoadFileResult {
     // Successfully opened the file
     LOAD_SUCCESS = 1,