Recurrence module

[bart-krakowski]

Parent

#25

What to build

Extract recurrence expansion and editing into a proper module that plugs into the kernel's read and write pipelines. When registered, the module expands Master Events into ephemeral Occurrences within the current viewport and handles all recurrence-specific writes (edit-this, edit-this-and-following, delete-this, delete-all).

Also extend the `RecurrenceRule` model to cover the full UI-builder subset: add `byMonthDay` (supports negative values, e.g. `-1` = last day of month) and ordinal weekdays (e.g. `{ weekday: 2, nth: 2 }` = 2nd Tuesday, `{ weekday: 5, nth: -1 }` = last Friday).

Occurrences are ephemeral — derived in the projection, never stored. The Master Event carries the rule; Overrides and Exceptions live on the master's rule.

Acceptance criteria

• `RecurrenceModule` plugs into the kernel via the module API (Kernel + module registration API #28) and is listed as a registered module in the Calendar product factory
• A Master Event with a `RecurrenceRule` produces the correct Occurrences for the current viewport
• Editing a single Occurrence creates an Override keyed by original start (RECURRENCE-ID); other occurrences are unaffected
• Editing "this and following" splits the master at the occurrence boundary, producing a new master for the remainder
• Deleting a single Occurrence adds it to `exDates` (Exception); the series continues
• `byMonthDay` (including negative values) generates the correct occurrences for monthly/yearly rules
• Ordinal weekday (`{ weekday, nth }`) generates the correct occurrences (e.g. 2nd Tuesday, last Friday)
• `goToNextOccurrence` / `goToPreviousOccurrence` navigate correctly across the series
• Removing the `RecurrenceModule` from the module list causes recurrence-related code to tree-shake out of the bundle

Blocked by

#28