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>

CMakeLists.txt

@@ -0,0 +1,20 @@
+cmake_minimum_required(VERSION 3.28)
+
+project(fantemp LANGUAGES CXX)
+
+# libavr from a local checkout (LIBAVR_ROOT) or the forge; the toolchain
+# file comes from the same checkout via CMakePresets.json.
+include(FetchContent)
+if(NOT LIBAVR_ROOT AND DEFINED ENV{LIBAVR_ROOT})
+	set(LIBAVR_ROOT $ENV{LIBAVR_ROOT})
+endif()
+if(LIBAVR_ROOT)
+	FetchContent_Declare(libavr SOURCE_DIR ${LIBAVR_ROOT})
+else()
+	FetchContent_Declare(libavr GIT_REPOSITORY git@git.blackmark.me:avr/libavr.git GIT_TAG main)
+endif()
+FetchContent_MakeAvailable(libavr)
+
+add_executable(fantemp src/main.cpp)
+target_link_libraries(fantemp PRIVATE libavr)
+add_custom_command(TARGET fantemp POST_BUILD COMMAND ${CMAKE_SIZE} $<TARGET_FILE:fantemp>)