fix: enforce strict recurrence validation and fix period encoding

[allenporter]

Closes #70. Enforces strict recurrence validation rules and fixes period encoding issues under RFC 5545.

[codecov]

Codecov Report

❌ Patch coverage is 71.07438% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.74%. Comparing base (21e463f) to head (04c38ab).
⚠️ Report is 60 commits behind head on main.

Files with missing lines	Patch %	Lines
ical/store.py	51.92%	25 Missing ⚠️
ical/component.py	83.63%	9 Missing ⚠️
ical/types/period.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #627      +/-   ##
==========================================
- Coverage   97.57%   96.74%   -0.83%     
==========================================
  Files          57       59       +2     
  Lines        3553     3846     +293     
==========================================
+ Hits         3467     3721     +254     
- Misses         86      125      +39     

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