✨ feature: Add calendar layout and occurrence handling functionality

- Introduced `calendar-layout.php` to manage month grid generation, occurrence intersection checks, and segment assignments for events.
- Added `occurrences.php` for normalizing, validating, and displaying event occurrences, including migration from legacy formats.
- Implemented functions for parsing dates, validating occurrence ranges, and formatting occurrences for display.
- Established hooks for synchronizing event occurrence data upon saving posts and validating ACF repeater fields.

.gitignore

@@ -1,2 +1,3 @@
 notes/
 phpcs-results.txt
+tests/