Add node brownout and ident event constants (#454)

Adds constants for standard events:
- Power brownout below required by standard;
- Power brownout below required by node;
- Node ident button pressed.

src/openlcb/Defs.hxx

@@ -246,6 +246,21 @@ struct Defs
     /// station may react to this by restoring locomotive speed settings.
     static constexpr uint64_t CLEAR_EMERGENCY_STOP_EVENT = 0x010000000000FFFCULL;
 
+    /// "Power supply brownout detected below minimum required by standard"
+    /// This event can be generated when a node detects that the CAN bus power
+    /// has dropped below the minimum declared in the standard.
+    static constexpr uint64_t POWER_STANDARD_BROWNOUT_EVENT = 0x010000000000FFF0ULL;
+
+    /// "Power supply brownout detected below minimum required by node"
+    /// This event can be generated when a node detects that it has
+    /// insufficient power for normal operations.
+    static constexpr uint64_t NODE_POWER_BROWNOUT_EVENT = 0x010000000000FFF1ULL;
+
+    /// "Ident button combination pressed"
+    /// This event can be generated by a node when it is instructed to generate
+    /// an identification event.
+    static constexpr uint64_t NODE_IDENT_BUTTON_EVENT = 0x010000000000FE00ULL;
+
     /** Status of the pysical layer link */
     enum LinkStatus
     {