Problem: LuaLS reported undefined-field for `_raw` on RecurSpec and
param-type-mismatch for `last_day.day` in `advance_date` because
`osdate.day` infers as `string|integer`.
Solution: Add `_raw` to the RecurSpec class annotation and cast
`last_day.day` to integer in both `math.min` call sites.
Problem: the plugin has no concept of recurring tasks, which is
needed for habits and repeating deadlines.
Solution: add recur.lua with parse(), validate(), next_due(),
to_rrule(), and shorthand_list(). Supports named shorthands (daily,
weekdays, weekly, etc.), interval notation (Nd, Nw, Nm, Ny), raw
RRULE passthrough, and ! prefix for completion-based mode. Includes
day-clamping for month/year advancement.
