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/board.hpp

@@ -0,0 +1,68 @@
+#pragma once
+
+#include <libavr/libavr.hpp>
+
+// The board composition: every peripheral of the fan controller in one
+// place. ATmega328P at 16 MHz — thermistor divider on ADC0 (PC0), fan on
+// OC0B (PD5) at 50 kHz, console on the hardware UART.
+namespace app {
+
+using namespace avr::literals;
+
+using dev = avr::device<{.clock = 16_MHz}>;
+
+// Millisecond uptime from timer2 CTC (the fan owns timer0).
+class uptime {
+	static inline volatile std::uint64_t ms = 0;
+
+  public:
+	using ticker = dev::timer2<{.frequency = 1_kHz, .on_compare = [] { ms = ms + 1; }}>;
+
+	static std::uint64_t millis()
+	{
+		avr::irq::atomic_guard lock;
+		return ms;
+	}
+};
+
+// 1000-sample averaging window fed by the conversion interrupt.
+class sampler {
+	static inline volatile std::uint32_t sum = 0;
+	static inline volatile std::uint16_t count = 0;
+	static inline volatile std::uint16_t window = 0;
+	static inline volatile bool ready = false;
+
+	static constexpr std::uint16_t samples = 1000;
+
+  public:
+	using input = dev::adc<{.input = avr::adc::input_pin(0),
+	                        .trigger = avr::adc::trigger::free_running,
+	                        .on_conversion = [](std::uint16_t value) {
+		                        sum = sum + value;
+		                        count = count + 1;
+		                        if (count >= samples) {
+			                        window = static_cast<std::uint16_t>(sum / samples);
+			                        sum = 0;
+			                        count = 0;
+			                        ready = true;
+		                        }
+	                        }}>;
+
+	// The finished average (raw 10-bit), once per window.
+	static bool take(std::uint16_t &value)
+	{
+		avr::irq::atomic_guard lock;
+		if (!ready)
+			return false;
+		value = window;
+		ready = false;
+		return true;
+	}
+};
+
+using fan = dev::pwm<avr::pd5, {.frequency = 50_kHz}>;
+
+using serial_t = dev::uart0<{.baud = 115200_Bd, .rx_buffer = 32, .max_baud_error = 2.5_pct}>;
+inline constexpr serial_t serial{};
+
+} // namespace app