build: the libavr pin advances past the audit sweep, and the numbers get names

The pin crosses libavr's phase-6 close and the guideline sweep behind it;
the image is byte-identical in both modes at 8206 bytes.

The port's own sweep, against the same rules. Every mutable `static inline`
takes `m_` - uptime's counter, the sampler's window, the controller's five,
the statistics histogram and the terminal's line state (rule 46; a private
`static constexpr` is a constant rather than state and keeps its bare name).
The command table is `std::to_array` and the serial config breaks one member
per line (rules 36, 40). And three numbers get the name they already had
somewhere: duty goes through `percent_t::of()` rather than a hand-built
basis-point count, the ADC's top count is `thermistor::adc_full_scale`
instead of 1023 in four places, and the two `0xffffffff` are `open_circuit`
- which was already declared five lines away - and `never_written`, which
replaces a comment explaining the literal (rules 5, 6, 41).

Measured, not assumed: rendering `adc_full_scale` into the `show` line
instead of leaving it in the message string cost 6 bytes, so the display
text stays text.

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

test/CMakeLists.txt

@@ -1,6 +1,6 @@
 # The board has no reset line and no programming header, so the loader-entry
 # route in the emitted image is the only thing standing between a firmware change
-# and an unreflashable board. It has been wrong before - see the script.
+# and an unreflashable board.
 find_package(Python3 COMPONENTS Interpreter)
 if(Python3_FOUND)
 	add_test(NAME fantemp.reachability