Rewrite on libavr

Same driver surface as the yazoalfa version — clock and alarm get/set,
alarm interrupts, flag check/clear — plus oscillator-stop detection and
die temperature. One source for tiny85 (software I2C) and mega328P (TWI),
built against libavr in both generated and reflect mode, byte-identical
.text across modes. Errors surface as std::expected instead of being
dropped; weekday-rate alarms now really set the DY bit (legacy cleared
it); multi-register access is one coherent bus transaction. 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": "attiny85-generated",
+			"inherits": "base",
+			"cacheVariables": { "LIBAVR_MCU": "attiny85", "LIBAVR_REFLECT": "OFF" }
+		},
+		{
+			"name": "attiny85-reflect",
+			"inherits": "base",
+			"cacheVariables": { "LIBAVR_MCU": "attiny85", "LIBAVR_REFLECT": "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": "attiny85-generated", "configurePreset": "attiny85-generated" },
+		{ "name": "attiny85-reflect", "configurePreset": "attiny85-reflect" },
+		{ "name": "atmega328p-generated", "configurePreset": "atmega328p-generated" },
+		{ "name": "atmega328p-reflect", "configurePreset": "atmega328p-reflect" }
+	]
+}