refactor: one alarm encoder, named register bits, and the example 14 B smaller

Both alarms' registers come from one encoder that derives DY/DT and the
mask bits from the rate, and the battery checks the bytes each Table 2
row writes instead of restating the enum values. Register addresses and
bits are named, the year register is the year less 2000 rather than a
division, and an alarm day reads back with one decode for both forms.
Over the whole API the image is 10 B smaller on both parts; a rate only
known at run time costs set_alarm1 6 B more.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -14,7 +14,7 @@ auto now = rtc::read_clock();       // result<date_time>
 auto armed = rtc::set_alarm1({}, ds3231::alarm1_rate::once_per_second);
 ```
 
-`example/main.cpp` is the full tour. libavr rides as the `libavr/` submodule,
+`example/main.cpp` is a complete program. libavr rides as the `libavr/` submodule,
 pinned to the commit this driver builds against; `LIBAVR_ROOT` (cache or
 environment) overrides it for development against a working tree:
 
@@ -26,9 +26,10 @@ cmake --build --preset attiny85-generated
 
 Presets cover attiny85/atmega328p in both libavr modes (generated and
 reflect); `ctest` runs the consteval battery over the BCD, the hours
-register's two formats, the weekday and the alarm rate encodings, and holds
-the example to its stated size — **1480 B** on the mega328P and **1270 B**
-on the tiny85, byte-identical across the two modes. The legacy
+register's two formats, the weekday, the alarm registers each rate writes and
+the control and status write-backs, and holds the example to its stated size —
+**1468 B** on the mega328P and **1256 B** on the tiny85, byte-identical across
+the two modes. The legacy
 yazoalfa-based driver lives on the `master` branch.
 
 The weekday register is derived from the date by default. A program that
@@ -91,6 +92,6 @@ One reference describes one part, hence `--project`. Release is what the gate
 compares, the presets defining no debug build; Debug carries the `-Og
 -gdwarf-4` pair libavr's own debug preset uses.
 
-Differences from legacy: weekday-rate alarms now actually set the DY bit
-(the old `setAlarmHelper` always cleared it), reads/writes are single
-coherent bus transactions, and errors are reported instead of ignored.
+Differences from legacy: weekday-rate alarms set the DY bit (the legacy
+driver always cleared it), each register block is read or written in one
+coherent bus transaction, and errors are reported instead of ignored.