EU regions merge (#10675)

* stronger together

* validate 2.4ghz regions

* less noise

* you're right, and that shapens the analysis significantly

* sassy rejoinder

src/mesh/RadioInterface.h

@@ -143,6 +143,10 @@ class RadioInterface
 
     virtual bool wideLora() { return false; }
 
+    /// Whether the radio can tune sub-GHz bands. False for 2.4 GHz-only chips (SX128x);
+    /// multiband chips like the LR1121 keep the default.
+    virtual bool supportsSubGhz() { return true; }
+
     /// Prepare hardware for sleep.  Call this _only_ for deep sleep, not needed for light sleep.
     virtual bool sleep() { return true; }
 
@@ -244,7 +248,12 @@ class RadioInterface
 
     static bool checkOrClampConfigLora(meshtastic_Config_LoRaConfig &loraConfig, bool clamp);
 
-    // Check if a candidate region is compatible and valid.
+    // Check if a candidate region is compatible and valid, with no side effects (safe for
+    // speculative UI checks). errBuf, if given, receives the failure reason.
+    static bool checkConfigRegion(const meshtastic_Config_LoRaConfig &loraConfig, char *errBuf = nullptr, size_t errLen = 0);
+
+    // Check if a candidate region is compatible and valid. On failure, logs at ERROR,
+    // records a critical error, and sends a client notification.
     static bool validateConfigRegion(const meshtastic_Config_LoRaConfig &loraConfig);
 
     // Check if a candidate radio configuration is valid.
@@ -253,6 +262,11 @@ class RadioInterface
     // Make a candidate radio configuration valid, even if it isn't.
     static void clampConfigLora(meshtastic_Config_LoRaConfig &loraConfig);
 
+    // If preset is locked to a sibling of currentRegion among the swappable EU regions
+    // (EU_868/EU_866/EU_N_868), return the sibling region owning the preset, else nullptr.
+    static const RegionInfo *regionSwapForPreset(meshtastic_Config_LoRaConfig_RegionCode currentRegion,
+                                                 meshtastic_Config_LoRaConfig_ModemPreset preset);
+
   protected:
     int8_t power = 17; // Set by applyModemConfig()