console: values in a column, the curve as a graph, and Ctrl+C

Three more things the original did better, and two bugs found doing them.

`curve` walks every whole degree from 10 to 60 with a bar, which is the
original's. The port sampled it every five degrees and printed a bare
percentage — ten numbers for a cubic, showing none of its shape. The bar
is the duty itself, so it needs no scale.

`show` and `statistics` print one value per line behind a dotted label
instead of a run-on line. That reads the same either way for a single
reading and is the whole difference when `monitor` emits one a second
forever. The label renderer is now shared with the help, since it is the
same thing three times; the flash overload takes its width from the
string's type, so the padding needs no hand-counted constant and the
labels stay out of SRAM. `statistics` gains the sample total, and says
"not available" rather than a zero it never measured.

Ctrl+C echoes `^C` and gives a fresh prompt, abandoning whatever was
half-typed, and it is what stops `monitor` now. Stopping on *any* byte
was the port's own invention and it reads fine until a host sends a line
ending: `monitor\r\n` stopped itself on the `\n` it arrived with, one
reading in, which is why monitoring looked broken from a script and fine
by hand.

The other bug is arithmetic. A temperature's fraction came from
`(quarters % 4) * 25`, and C++ gives a negative remainder for a negative
dividend — so -40.25 C printed as "-40.-25". The sign comes off first
now, and the fraction is two digits, so the column lines up: -40.00,
-40.25.

Verified against v1.8b on the board, which was flashed back to compare
against directly: same 51 curve rows over the same span with the same
100-column bars, agreeing within the one percentage point the consteval
table costs against the legacy runtime doubles.

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

README.md

@@ -1,6 +1,6 @@
 # fantemp
 
-**v2.1.** Temperature-controlled fan firmware (ATmega328P, 16 MHz), rewritten on
+**v2.2.** Temperature-controlled fan firmware (ATmega328P, 16 MHz), rewritten on
 [libavr](https://git.blackmark.me/avr/libavr): thermistor on ADC0 sampled
 free-running and averaged over 1000 conversions, fan on OC0B at 50 kHz,
 115200 Bd serial console (`help` lists the commands), temperature
@@ -21,6 +21,17 @@ command that cannot be abbreviated, because `r` should not be able to wipe the
 histogram. `save` (new) forces a writeback, which otherwise happens every 30
 minutes and on the way into the bootloader.
 
+`show`, `statistics` and the histogram print one value per line behind a dotted
+label, the way the original did — a run-on line is fine for one reading and
+unreadable when `monitor` emits one a second. `curve` walks every whole degree
+from 10 to 60 with a bar, because the curve is a cubic and five-degree samples
+without a graph show none of its shape.
+
+**Ctrl+C** abandons a half-typed line and gives a fresh prompt, echoing `^C`, and
+it is what stops `monitor`. Stopping on *any* byte, which is what the port did
+first, reads well right up until a host sends a line ending: `monitor\r\n` then
+stopped itself on the `\n` it arrived with, one reading in.
+
 ## Reaching the bootloader
 
 `bootloader` **jumps**; it does not reset. That is not a style choice: