build: the libavr pin advances past phase 6, and the driver takes the bus whole

The register sugar now rides the bus's own master role - device<Bus>
resolves avr::i2c::device<typename Bus::master, bus_address>, so a
consumer keeps handing over its declared dev::i2c<...> unchanged - and
the DS3231's hardwired 0x68 is a named constant. The example's four
discards became the LED's own error signal (a failed seed or a failed
alarm clear holds it dark, the same word a stuck bus says), the tree is
reformatted under InsertBraces, the sources are ASCII, and the README's
stale Studio byte counts are replaced by the claim its check-flags gate
holds.

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

README.md

@@ -11,7 +11,7 @@ using bus = dev::i2c<{.frequency = 100_kHz}>;
 using rtc = ds3231::device<bus>;
 
 auto now = rtc::read_clock();       // result<date_time>
-(void)rtc::set_alarm1({}, ds3231::alarm1_rate::once_per_second);
+auto armed = rtc::set_alarm1({}, ds3231::alarm1_rate::once_per_second);
 ```
 
 `example/main.cpp` is the full tour. libavr rides as the `libavr/` submodule,
@@ -31,9 +31,9 @@ reflect). The legacy yazoalfa-based driver lives on the `master` branch.
 
 `master` carries a Studio solution, so this branch does too: `ide/ds3231.atsln`
 builds `example/main.cpp` for both parts the presets cover, each to a
-**byte-identical `.text`** against the CMake build — 1242 B on the ATmega328P,
-1212 B on the ATtiny85 — with the flags mirrored by hand. CMake remains the
-build system.
+**byte-identical `.text`** against the CMake build (`check-flags.py` below is
+what holds the flag sets equal, so the sizes are the presets' own), with the
+flags mirrored by hand. CMake remains the build system.
 
 `avrdevice` is a project-level property in Studio, so a part means a project, not
 a configuration: `ide/atmega328p/` and `ide/attiny85/`. They need separate