build: the libavr pin advances, and the tiny85 image grows 78 B

43fc479 -> aec9955. The mega328P does not move at 1480 B - it drives the RTC
from the TWI block, which neither change touches. The tiny85 has no TWI, so
dev::i2c resolves to the bit-banged master there and takes both:

  cb40a13, the bit period                      1238 -> 1262  (+24)
  943ffbe, one call shape over every master    1262 -> 1316  (+54)

The first is a bug being paid for. software_master delayed a fixed quarter
period and let its own instructions make up the rest of the bit, so the rate
on the wire followed the inliner; each bit now delays a low and a high phase
net of what that phase already spends. The DS3231 is a 400 kHz part and this
example asks for 100, so nothing here was out of spec - the bytes buy a rate
that no longer moves when the image does.

The second is a trade, and it is worth writing down which way it went. The
shared ladder is smaller than the two it replaced - run_transfer is 232 B
against read_regs 162 + write_regs 102 + stop 30 - but the call-site thunks
that marshal spans into it cost 116 B where the specialised ladders were
inlined, so a driver making both calls nets +54. Filed upstream with these
numbers rather than worked around here: which way the trade falls is a
property of the consumer's mix, and that is the library owner's call.

Size re-recorded in the two places that state it. Four presets green,
cross-mode identity held on both chips.

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

README.md

@@ -27,7 +27,7 @@ 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 **1238 B**
+the example to its stated size — **1480 B** on the mega328P and **1316 B**
 on the tiny85, byte-identical across the two modes. The legacy
 yazoalfa-based driver lives on the `master` branch.