fix(kiss): periodic noise floor calibration and AGC reset

- Trigger noise floor calibration every 2s and AGC reset every 30s in main loop.
- Reorder loop to match Dispatcher: calibrate + radio.loop() before AGC reset
  and recvRaw() so RSSI is never sampled right after startReceive().
- Update protocol doc with calibration intervals and typical noise floor range.
- Variant platformio.ini updates (heltec_v3, rak4631).

variants/rak4631/platformio.ini

@@ -183,4 +183,13 @@ build_flags =
   -D MESH_DEBUG=1
 build_src_filter = ${rak4631.build_src_filter}
   +<helpers/ui/SSD1306Display.cpp>
-  +<../examples/simple_sensor>
+  +<../examples/simple_sensor>
+
+[env:RAK_4631_kiss_modem]
+extends = rak4631
+build_flags =
+  ${rak4631.build_flags}
+build_src_filter = ${rak4631.build_src_filter}
+  +<../examples/kiss_modem/>
+lib_deps =
+  ${rak4631.lib_deps}