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/main.cpp

@@ -0,0 +1,32 @@
+#include <ds3231/ds3231.hpp>
+#include <libavr/libavr.hpp>
+
+using namespace avr::literals;
+
+// One source for the tiny85 (open-drain software I2C on the USI pins) and
+// the mega328P (TWI hardware). On first power-up the clock is seeded and
+// alarm 1 armed; then the LED mirrors the seconds parity — a stuck LED
+// means bus errors.
+using dev = avr::device<{.clock = 8_MHz}>;
+using bus = dev::i2c<{.frequency = 100_kHz}>;
+using rtc = ds3231::device<bus>;
+using led = dev::output<avr::pb3>;
+
+int main()
+{
+	avr::init<bus, led>();
+
+	if (auto stopped = rtc::oscillator_stopped(); stopped.value_or(false)) {
+		(void)rtc::write_clock({{2026, 1, 1}, {12, 0, 0}});
+		(void)rtc::set_alarm1({}, ds3231::alarm1_rate::once_per_second);
+		(void)rtc::clear_oscillator_stopped();
+	}
+
+	while (true) {
+		if (auto now = rtc::read_clock(); now.has_value())
+			led::write(now->second % 2 == 0);
+		if (auto fired = rtc::alarm1_fired(); fired.value_or(false))
+			(void)rtc::clear_alarm1();
+		dev::delay<100_ms>();
+	}
+}