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>

.gitignore

@@ -1,11 +1,2 @@
-.vs
-Release
-Debug
-*.componentinfo.xml
-*.elf
-*.o
-*.hex
-*.srec
-*.eeprom
-*.lss
-*.map
+build/
+.cache/