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>

README.md

@@ -0,0 +1,22 @@
+# fantemp
+
+Temperature-controlled fan firmware (ATmega328P, 16 MHz), rewritten on
+[libavr](https://git.blackmark.me/avr/libavr): thermistor on ADC0 sampled
+free-running and averaged over 1000 conversions, fan on OC0B at 50 kHz,
+115200 Bd serial console (`help` lists the commands), temperature
+histogram persisted to EEPROM, watchdog-reset path into a boot-section
+bootloader.
+
+The Steinhart–Hart math of the legacy firmware (runtime doubles + libm
+log) is gone: the Beta equation and the cubic fan curve are evaluated
+consteval into flash tables — the firmware itself never touches floating
+point.
+
+Build against a libavr checkout:
+
+```sh
+LIBAVR_ROOT=/path/to/libavr cmake --preset atmega328p-generated
+cmake --build --preset atmega328p-generated
+```
+
+Legacy (yazoalfa submodules) stays on `master`.