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/curve.hpp

@@ -0,0 +1,49 @@
+#pragma once
+
+#include <array>
+#include <cstdint>
+
+#include <avr/pgmspace.h>
+
+// The auto-mode fan curve, tabulated at compile time: the legacy cubic
+// (0.002246·x³ − 0.09·x² + 0.91·x, zero below 20 °C) becomes a flash
+// lookup of duty percent per °C.
+namespace app::curve {
+
+namespace detail {
+
+consteval std::uint8_t duty_entry(int celsius)
+{
+	double x = celsius;
+	if (x < 20)
+		return 0;
+	double duty = 0.002246 * x * x * x - 0.09 * x * x + 0.91 * x;
+	if (duty < 0)
+		duty = 0;
+	if (duty > 100)
+		duty = 100;
+	return static_cast<std::uint8_t>(duty + 0.5);
+}
+
+struct table {
+	[[gnu::progmem]] static constexpr std::array<std::uint8_t, 100> data = [] {
+		std::array<std::uint8_t, 100> out{};
+		for (int t = 0; t < 100; ++t)
+			out[static_cast<std::size_t>(t)] = duty_entry(t);
+		return out;
+	}();
+};
+
+} // namespace detail
+
+// Duty percent for a temperature (clamped to the 0..99 °C table window).
+inline std::uint8_t duty(std::int8_t celsius)
+{
+	if (celsius < 0)
+		celsius = 0;
+	if (celsius > 99)
+		celsius = 99;
+	return pgm_read_byte(&detail::table::data[static_cast<std::uint8_t>(celsius)]);
+}
+
+} // namespace app::curve