fix: an alarm's interrupt could be turned on and never off, and nothing tested any of it

`set_alarm1/2`'s `enable_interrupt = false` did nothing at all - it skipped the
control-register write rather than clearing A1IE/A2IE - and there was no other
way to clear them, so an alarm armed once with its interrupt on drove INT/SQW
for good. It is `alarm_interrupt::off` now and it clears the bit, with
`listen_alarm1/2()` and `mute_alarm1/2()` beside it for a caller that arms once
and changes its mind later. The bare bool went with it (rule 37): `set_alarm1(at,
rate, false)` said nothing at the point of use.

`bool SetWeekday` was the same shape one level up - `device<bus, false>` names
nothing - and its two writers disagreed about what it meant: `write_date`
skipped the weekday register, `write_clock` stamped a placeholder 1 into it, so
a program that turned the maintenance off still had the register overwritten
and no way to set it. `weekday_source::external` now means the register is the
program's, in both writers.

test/consteval.cpp is the battery this driver never had, over the arithmetic
that has no bus in it: the BCD round trip across every representable value and
its nibble layout, the hours register in both formats including all four
noon/midnight cases, Sakamoto's weekday against six calendar dates (leap day,
the day after it, and 2100's missing leap), and both alarms' rate encodings
transcribed against 19-5170 Table 2 - including that alarm 2's masks sit one
place below alarm 1's, which is the property that makes one distribution loop
wrong for the other. Red-green: two assertions fire on a flipped DY bit.

Beside it, one decode rather than two: `read_clock` and `read_time` spelled the
seconds/minutes/hours triplet out separately (rule 6).

Every image byte-identical on both chips in both modes.

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

README.md

@@ -25,7 +25,26 @@ cmake --build --preset attiny85-generated
 ```
 
 Presets cover attiny85/atmega328p in both libavr modes (generated and
-reflect). The legacy yazoalfa-based driver lives on the `master` branch.
+reflect); `ctest` runs the consteval battery over the BCD, the hours
+register's two formats, the weekday and the alarm rate encodings. The legacy
+yazoalfa-based driver lives on the `master` branch.
+
+The weekday register is derived from the date by default. A program that
+assigns its own meaning to the device's 1..7 takes ownership of it, and no
+write here touches it:
+
+```cpp
+using rtc = ds3231::device<bus, ds3231::weekday_source::external>;
+```
+
+An alarm arms its INT/SQW output unless told otherwise, and can be muted and
+re-wired without re-arming:
+
+```cpp
+auto quiet = rtc::set_alarm2(at, ds3231::alarm2_rate::minutes_match,
+                             ds3231::alarm_interrupt::off);
+auto again = rtc::listen_alarm2();
+```
 
 ## Atmel Studio