[codeling]

Fixes a flaky failure of integration (PHP 8.3, MySQL 8.0) on #33. That PR's change (docker/login-action) isn't run by any test, and the same commit passed in its push run:

not ok 38 - creating an entry with an existing tag finds the tag by its index
  error: 'enter read 500 rows, at most 150 expected (table size 10000)'

Cause

The 500 rows aren't enter's own; locally it reads exactly 3, every time. They are the rows of SELECT COUNT(*) FROM todo WHERE deleted=1 AND list_id=10, which the empty-trash test runs right after its measurement (list 10 has 500 entries). rowsRead() waits until Threads_connected is 1, but MySQL can add a closed connection's counters to the global ones a moment after that. On the slower CI runner, the counters were read in that gap, so those 500 rows only showed up during the next measurement.

My first guess, that the overhead of reading the counters varies, was wrong: on MySQL 8.0.46 it is exactly 1000 rows in 40 of 40 samples. The first commit of this PR was based on that guess; the second one replaces it.

Change (tests/performance.test.js)

• settledRowsRead(): before and after the request, the counters are read until two readings in a row differ by the overhead alone, so late counters are no longer counted for the request.
• The overhead is the smallest difference of 3 pairs of readings, since late counters can only make a pair larger. Every reading after before adds one overhead, and readings counts them so each one is subtracted.
• The limits are unchanged, so a full scan still fails the test.

Verified

• MySQL 8.0.46 (the image CI uses), 3 runs: all 7 measurements give the same numbers as the old code (enter: 3, empty-trash: 502, …), no failures.
• Simulated race (500 rows that show up only after the 3rd reading of the enter test): the old test fails with enter: 503 rows read, like in CI; the new one reports enter: 3. The real race didn't happen on my machine (0 in 60 tries), so this simulation is the evidence that it's fixed.
• MariaDB, full npm test: 70/70 pass.

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