test: pin group-events-by-day clock and add midnight edge cases

The Normal and Boundary tests built events as fixed-minute offsets from real current-time. On late-night runs the +60 minute event in single-day crossed midnight and the test failed because events 1-2 landed "Today" while event 3 landed "Tomorrow". The intent of these Normal tests is routine grouping, not midnight behavior, so they should be deterministic.

Pin the clock with with-test-time using the established let-bind pattern. with-test-time re-evaluates BASE-TIME inside the mocked current-time lambda, so passing test-time-today-at directly would recurse, because the helper itself calls current-time.

Add two new Boundary tests that explicitly exercise the day-crossing logic at 23:50. One event at +5 minutes stays in Today and one at +20 minutes crosses into Tomorrow. These are the first tests in the file that actually target midnight behavior.

Drop stale "REFACTORED: Uses dynamic timestamps" annotations from docstrings since the behavior is documented at the file level.

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