Rewrite on libavr

Same driver surface as the yazoalfa version — clock and alarm get/set,
alarm interrupts, flag check/clear — plus oscillator-stop detection and
die temperature. One source for tiny85 (software I2C) and mega328P (TWI),
built against libavr in both generated and reflect mode, byte-identical
.text across modes. Errors surface as std::expected instead of being
dropped; weekday-rate alarms now really set the DY bit (legacy cleared
it); multi-register access is one coherent bus transaction. Legacy stays
on master.

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

example/CMakeLists.txt

@@ -0,0 +1,3 @@
+add_executable(clock main.cpp)
+target_link_libraries(clock PRIVATE ds3231)
+add_custom_command(TARGET clock POST_BUILD COMMAND ${CMAKE_SIZE} $<TARGET_FILE:clock>)