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>

.gitignore

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