Rewrite on libavr

Same controller: thermistor on ADC0 averaged over 1000 free-running
conversions, 50 kHz fan PWM on OC0B, 115200 Bd console with the full
command set, EEPROM temperature histogram, watchdog-reset path into the
boot section. The Steinhart-Hart math and the libm log are gone — the
Beta equation and the cubic fan curve are consteval-evaluated into
flash tables; the firmware never does floating point. Byte-identical
.text in both libavr modes. Legacy stays on master.

Co-Authored-By: Claude <noreply@anthropic.com>

.gitmodules

@@ -1,18 +0,0 @@
-[submodule "fantemp/uart"]
-	path = fantemp/uart
-	url = git@git.blackmark.me:avr/uart.git
-[submodule "fantemp/flash"]
-	path = fantemp/flash
-	url = git@git.blackmark.me:avr/flash.git
-[submodule "fantemp/io"]
-	path = fantemp/io
-	url = git@git.blackmark.me:avr/io.git
-[submodule "fantemp/adc"]
-	path = fantemp/adc
-	url = git@git.blackmark.me:avr/adc.git
-[submodule "fantemp/type"]
-	path = fantemp/type
-	url = git@git.blackmark.me:avr/type.git
-[submodule "fantemp/eeprom"]
-	path = fantemp/eeprom
-	url = git@git.blackmark.me:avr/eeprom.git