Closes incidencia points D + E:
- Heading collapses the shared year when both visible months are the
same year: "May – June 2026" instead of "May 2026 – June 2026".
Cross-year still renders both ends explicitly ("June 2025 – July
2026"). Implemented in `RangeCalendarProvider.headingValue`.
- date-range-picker demo: drop the per-month title that previously
rendered BELOW each grid (duplicate of the main heading). Instead,
render one centered title per visible calendar INSIDE the header
bar between the prev/next buttons. The grid below now shows just
the weeks. A `range-header-titles` grid container splits the
available header space equally across `month-count` titles, so
each label sits centered over its calendar grid.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
dev
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