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>

CMakePresets.json

@@ -0,0 +1,43 @@
+{
+	"version": 8,
+	"configurePresets": [
+		{
+			"name": "base",
+			"hidden": true,
+			"generator": "Ninja",
+			"binaryDir": "${sourceDir}/build/${presetName}",
+			"toolchainFile": "$env{LIBAVR_ROOT}/cmake/avr-toolchain.cmake",
+			"cacheVariables": {
+				"CMAKE_BUILD_TYPE": "Release",
+				"CMAKE_EXPORT_COMPILE_COMMANDS": "ON",
+				"CMAKE_COLOR_DIAGNOSTICS": "ON"
+			}
+		},
+		{
+			"name": "atmega328p-generated",
+			"inherits": "base",
+			"cacheVariables": {
+				"LIBAVR_MCU": "atmega328p",
+				"LIBAVR_REFLECT": "OFF"
+			}
+		},
+		{
+			"name": "atmega328p-reflect",
+			"inherits": "base",
+			"cacheVariables": {
+				"LIBAVR_MCU": "atmega328p",
+				"LIBAVR_REFLECT": "ON"
+			}
+		}
+	],
+	"buildPresets": [
+		{
+			"name": "atmega328p-generated",
+			"configurePreset": "atmega328p-generated"
+		},
+		{
+			"name": "atmega328p-reflect",
+			"configurePreset": "atmega328p-reflect"
+		}
+	]
+}