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>

src/main.cpp

@@ -0,0 +1,29 @@
+#include <libavr/libavr.hpp>
+
+#include "board.hpp"
+#include "bootloader.hpp"
+#include "controller.hpp"
+#include "statistics.hpp"
+#include "terminal.hpp"
+
+using namespace app;
+
+template struct avr::isr::emit<uptime::ticker, sampler::input, serial_t>;
+
+int main()
+{
+	bootloader::handle_reset();
+
+	avr::init<uptime::ticker, sampler::input, fan, serial_t, statistics>();
+	avr::irq::enable();
+	sampler::input::start();
+	controller::init();
+	terminal::init();
+
+	while (true) {
+		controller::poll();
+		if (controller::data_available())
+			statistics::record(static_cast<std::int8_t>((controller::temperature_quarters() + 2) / 4));
+		terminal::poll();
+	}
+}