docs: the stopwatch, and why its reading is floored and banked

`ARCHITECTURE.md` gains §15 for the stopwatch and records the one correctness
find this slice turned up: a lap taken inside a segment a reboot later
discarded would have dragged the readout, the lap totals and the shade
backwards. The reading is floored at the last lap's total — and the floor is
banked, not merely displayed, or the shade's free-running chronometer walks
away from a tab that is standing still.

The package, module, receiver, service and permission tables all count one
more; §10 loses the two rows M7 closed.

CHANGELOG.md

@@ -138,3 +138,19 @@ Tag sections feed the release notes — see [`docs/RELEASING.md`](docs/RELEASING
 - Progress on each timer card is drawn by the Expressive wavy indicator, and the
   wave itself is the state — it flattens when you pause and when the timer is
   done.
+- The stopwatch tab, for real: start, pause and reset, with a readout down to
+  hundredths of a second that stays legible because the fraction is drawn
+  smaller than the seconds rather than shouting alongside them.
+- Laps, each with its own split and its running total, newest at the top, and
+  the fastest and slowest of them marked — in colour and in words, so a screen
+  reader hears the distinction too. The lap you are in the middle of has its own
+  row, counting up.
+- An ongoing notification with Lap and Pause in the shade, which costs no
+  battery to keep accurate because the system draws the ticking; Clockula posts
+  once when something actually changes.
+- A stopwatch that keeps counting while the app is closed, and that comes back
+  after a restart paused — with every lap intact — at the time it had banked,
+  rather than inventing the time it lost. Changing the system clock, in either
+  direction, cannot move it at all.
+- The app's big numbers now sit in fixed columns, so a readout no longer jitters
+  as its digits change — on every screen that shows one, not only the stopwatch.