`Sequencer.reentered` is a counter nothing reads

[bdbarnett]

The re-entrancy guard counts refused ticks and no caller, test or screen looks
at it. Either surface it where a user would see a dropped step, or drop it.

Update at merge: on the ESP32-P4 the counter read 4 over a twenty-second full bar at 200 BPM — the guard does fire on a board, driven by the board's own scheduler. The drum machine still shows it nowhere.

From #91.

[bdbarnett]

Kept, and given a reader. dac55d3.

Why not dropped. The board settled it: the counter read 4 over a twenty-second full bar at 200 BPM on the P4, so the guard fires in real use and a number nobody can see is the wrong answer.

Why not "where a user sees a dropped step" either. reentered is not a dropped step. A refused tick costs nothing — the look-ahead is hundreds of milliseconds and the next tick lays what this one did not — so an app that showed it as a missed note would be lying about its own health. What it means is that the timer is arriving faster than a bar can be laid, and the cure is a slower timer or a deeper ahead.

So it goes on a health surface next to the counter that is a dropped step. Sequencer.health() returns one dict an app can put on a screen:

scheduled   events laid on the queue, ever
refused     events the queue turned away: steps not heard
cancelled   events taken back by a tempo change or stop()
reentered   ticks that arrived while this one was writing
needed      the depth these tracks want (depth_needed())
capacity    the depth this queue has, or None

refused is the one that means a step was not heard, and as of #93 it also has a warning of its own — start() says so before anything is dropped, and the first refusal says so again. Each entry's meaning is on health()'s docstring, so a reader does not have to guess which of the four is bad news.

Guarded by test_health_carries_the_counter_that_had_no_reader, which drives an interrupt into a full bar and asserts reentered climbs while refused stays 0 at the app's own capacity.

Noted on #38, which is the same shape at the pump level and not this repository's.