Closes 2 bugs from the date-range-picker incidencia 2026-05-20:
#1 — `RangeCalendarProvider` no longer auto-shifts the placeholder when
a selection lands in the last visible month. The visible months stay
where the user put them; navigation is explicit (prev/next/month/year
controls or keyboard). `reanchorInitialSelection` is removed and its
`shift-navigate` trigger goes with it.
#2 — clicking an endpoint of a completed range now drops only that
endpoint and re-anchors on the surviving one. The previous behavior
cleared both endpoints, which forced users to rebuild the entire range
to amend it. The provider already implemented this; only the test
codified the old behavior. Test rewritten to match the documented
intent and symmetric for start/end.
The two range-calendar tests that previously asserted the wrong
behavior now cover the correct invariants.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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