fix: the quarter-degree rounding was wrong below zero, in three copies

`(quarters + 2) / 4` is round-half-up only for positive values: C truncates a
negative quotient toward zero, so -3.00 C read as -2, -1.00 C as 0 and -0.75 C
as 0 - nine of fifteen negative quarter-values off by a whole degree, always
toward zero. `(quarters + 2) >> 2` is an arithmetic shift, which floors, and
is right across the whole range. It is also smaller: the shift skips the bias
correction signed division needs.

The formula was written three times - thermistor::celsius(), which nothing
called, and inline at both live call sites - so the defect had three homes and
so would its fix. One `thermistor::whole_degrees()` now, called from both, and
the dead entry point is gone (rules 6, 27).

test/consteval.cpp is new and is what should have caught it: whole_degrees
across zero including both ties and both table limits, the curve's start and
saturation points and its monotonicity, and the thermistor table anchored
where the Beta equation fixes it - the count at which the divider reads the
thermistor's nominal resistance must read the nominal temperature - plus both
clamps and the fall across every step. Red-green: four assertions fire against
the old division.

Beside it: the cubic's three coefficients are named rather than inlined and
restated in prose (rule 5), the consteval table builders take explicit 32-bit
types (rule 25), the curve's clamp reads the table's own size (rule 36), and
the serial override says what expects the rate rather than what the board has
always done (rules 12, 13).

And bootloader::handle_reset()'s watchdog diversion is gone. Its own comment
called it a leftover "kept only because it is free and cannot hurt", and it
did not: pureboot peeks WDRF without clearing it and hands back on purpose, so
a watchdog reset arrives here with the flag still set and the diversion jumped
into the loader with MCUSR already cleared - opening the activation window
that policy exists to close. Clearing MCUSR is the whole job and stays.

8206 -> 8168 bytes, byte-identical between generated and reflect.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/board.hpp

@@ -67,8 +67,8 @@ class sampler {
 using fan = dev::pwm<avr::pd5, {.frequency = 50_kHz}>;
 
 // 115200 at 16 MHz lands +2.1 % off, past the receiver-tolerance table the
-// solver holds rates to - the rate this board has always spoken, so the
-// override states that it is meant.
+// solver holds rates to. It is the rate the console on the other end of the
+// cable expects, so the override states that the miss is meant.
 using serial_t = dev::uart0<{
     .baud = 115200_Bd,
     .rx_buffer = 32,

src/bootloader.hpp

@@ -50,19 +50,16 @@ class bootloader {
 	}
 
   public:
-	// Call first thing in main. consume_reset_cause() reads *and clears* MCUSR, which
-	// matters on its own: a lingering WDRF forces the watchdog back on at its
-	// shortest timeout. The diversion below is a leftover of the legacy route
-	// and is kept only because it is free and cannot hurt - with BOOTRST
-	// programmed the loader has already run before this line, so nothing
-	// normally reaches it.
+	// Call first thing in main. Clearing MCUSR is the whole job: a lingering
+	// WDRF forces the watchdog back on at its shortest timeout, which is a
+	// reset loop rather than a boot. The cause is not routed on - pureboot
+	// peeks WDRF without clearing it and hands back on purpose, so a watchdog
+	// reset arrives here with the flag still set, and diverting into the
+	// loader on it would reopen the very window that policy closes.
 	static void handle_reset()
 	{
-		auto cause = avr::power::consume_reset_cause();
+		avr::power::consume_reset_cause();
 		guard::disable();
-		if (cause.watchdog && present()) {
-			call(reinterpret_cast<jump_fn>(base / 2));
-		}
 	}
 
 	// Hand over for real: no reset, so no WDRF for the loader to refuse.

src/controller.hpp

@@ -33,7 +33,7 @@ class controller {
 		m_temp_quarters = thermistor::quarters(sample);
 		m_have_data = true;
 		if (m_auto_mode) {
-			m_percent = curve::duty(static_cast<std::int8_t>((m_temp_quarters + 2) / 4));
+			m_percent = curve::duty(thermistor::whole_degrees(m_temp_quarters));
 		}
 		fan::set_duty(avr::percent_t::of(m_percent));
 	}

src/curve.hpp

@@ -5,20 +5,25 @@
 
 #include <libavr/flash.hpp>
 
-// The auto-mode fan curve, tabulated at compile time: a cubic in C
-// (0.002246*x^3 - 0.09*x^2 + 0.91*x, zero below 20 C) as a flash_table of
-// duty percent per C.
+// The auto-mode fan curve, tabulated at compile time as a flash_table of duty
+// percent per C: a cubic that is held at zero below the temperature the fan
+// starts at.
 namespace app::curve {
 
+inline constexpr double cubic_term = 0.002246;
+inline constexpr double square_term = -0.09;
+inline constexpr double linear_term = 0.91;
+inline constexpr std::int8_t start_celsius = 20;
+
 namespace detail {
 
-consteval std::uint8_t duty_entry(int celsius)
+consteval std::uint8_t duty_entry(std::int32_t celsius)
 {
 	double x = celsius;
-	if (x < 20) {
+	if (x < start_celsius) {
 		return 0;
 	}
-	double duty = 0.002246 * x * x * x - 0.09 * x * x + 0.91 * x;
+	double duty = cubic_term * x * x * x + square_term * x * x + linear_term * x;
 	if (duty < 0) {
 		duty = 0;
 	}
@@ -30,7 +35,7 @@ consteval std::uint8_t duty_entry(int celsius)
 
 inline constexpr avr::flash_table<[] {
 	std::array<std::uint8_t, 100> out{};
-	for (int t = 0; t < 100; ++t) {
+	for (std::int32_t t = 0; t < static_cast<std::int32_t>(out.size()); ++t) {
 		out[static_cast<std::size_t>(t)] = duty_entry(t);
 	}
 	return out;
@@ -39,14 +44,15 @@ inline constexpr avr::flash_table<[] {
 
 } // namespace detail
 
-// Duty percent for a temperature (clamped to the 0..99 C table window).
+// Duty percent for a temperature, clamped to the table's own window.
 inline std::uint8_t duty(std::int8_t celsius)
 {
+	constexpr auto highest = static_cast<std::int8_t>(detail::table.size() - 1);
 	if (celsius < 0) {
 		celsius = 0;
 	}
-	if (celsius > 99) {
-		celsius = 99;
+	if (celsius > highest) {
+		celsius = highest;
 	}
 	return detail::table[static_cast<std::uint8_t>(celsius)];
 }

src/main.cpp

@@ -5,6 +5,7 @@
 #include "controller.hpp"
 #include "statistics.hpp"
 #include "terminal.hpp"
+#include "thermistor.hpp"
 
 using namespace app;
 
@@ -23,7 +24,7 @@ int main()
 	while (true) {
 		controller::poll();
 		if (controller::data_available()) {
-			statistics::record(static_cast<std::int8_t>((controller::temperature_quarters() + 2) / 4));
+			statistics::record(thermistor::whole_degrees(controller::temperature_quarters()));
 		}
 		terminal::poll();
 	}

src/thermistor.hpp

@@ -34,7 +34,7 @@ consteval double temperature_of(double adc)
 // 256 entries over the 10-bit range (steps of 4 counts), quarter- C,
 // clamped to a sane sensor window; entry 256 mirrors 255 so interpolation
 // at full scale has a right neighbour.
-consteval std::int16_t quarters_entry(int index)
+consteval std::int16_t quarters_entry(std::int32_t index)
 {
 	double adc = index * 4.0;
 	if (adc < 4) {
@@ -55,7 +55,7 @@ consteval std::int16_t quarters_entry(int index)
 
 inline constexpr avr::flash_table<[] {
 	std::array<std::int16_t, 257> out{};
-	for (int i = 0; i < 257; ++i) {
+	for (std::int32_t i = 0; i < static_cast<std::int32_t>(out.size()); ++i) {
 		out[static_cast<std::size_t>(i)] = quarters_entry(i < 256 ? i : 255);
 	}
 	return out;
@@ -74,9 +74,12 @@ inline std::int16_t quarters(std::uint16_t adc)
 	return static_cast<std::int16_t>(a + ((b - a) * frac) / 4);
 }
 
-inline std::int8_t celsius(std::uint16_t adc)
+// Whole degrees from quarter-degrees, rounded to nearest. The shift is the
+// point: C truncates a negative quotient toward zero, so `(q + 2) / 4` reads
+// -3.00 C as -2, where an arithmetic shift floors and reads it as -3.
+inline constexpr std::int8_t whole_degrees(std::int16_t quarters)
 {
-	return static_cast<std::int8_t>((quarters(adc) + 2) / 4);
+	return static_cast<std::int8_t>((quarters + 2) >> 2);
 }
 
 } // namespace app::thermistor