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/heltec_v3/platformio.ini

@@ -367,3 +367,12 @@ build_src_filter = ${Heltec_lora32_v3.build_src_filter}
 lib_deps =
   ${Heltec_lora32_v3.lib_deps}
   ${esp32_ota.lib_deps}
+
+[env:Heltec_v3_kiss_modem]
+extends = Heltec_lora32_v3
+build_flags =
+  ${Heltec_lora32_v3.build_flags}
+build_src_filter = ${Heltec_lora32_v3.build_src_filter}
+  +<../examples/kiss_modem/>
+lib_deps =
+  ${Heltec_lora32_v3.lib_deps}