feat: parse vjournal components

[redryerye]

Summary

• Add an ICJournal model for parsed VJOURNAL components
• Parse common journal fields including UID, DTSTAMP, DTSTART, SUMMARY, DESCRIPTION, and STATUS
• Expose parsed journals on ICalendar.journals
• Mark VJOURNAL parsing complete in the README roadmap

Example ICS

BEGIN:VJOURNAL
UID:journal-test
DTSTAMP:20240728T120000Z
DTSTART;VALUE=DATE:20240729
SUMMARY:Daily note
DESCRIPTION:Retrospective notes
STATUS:FINAL
END:VJOURNAL

What becomes available

let journal = calendar.journals.first
journal?.uid // "journal-test"
journal?.summary // "Daily note"
journal?.dtStart?.type // .date

Validation

• swift test
• swiftlint lint --quiet