v4.0.0 - LocalXxx to PlainXxx; add 2025 versions of zonedb; replace 'fold'

• 4.0.0 (2025-10-21, TZDB version 2025b)
  • See MIGRATING.md on breaking API changes, and how to
    migrate.
  • Breaking Rename LocalXxx to PlainXxx, following the conventions
    used by more modern timezone libraries (JavaScript Temporal, Python
    whenever).
    • LocalDate -> PlainDate
    • LocalDateTime -> PlainDateTime
    • LocalTime -> PlainTime
    • Backwards compatibility macros and methods have been added:
      • #define LocalDate PlainDate
      • #define LocalDateTime PlainDateTime
      • #define LocalTime PlainTime
  • Breaking Rename various methods with 'Local' to 'Plain':
    • ZonedDateTime::localDateTime() -> plainDateTime()
    • ZonedDateTime::forLocalDateTime() -> forPlainDateTime()
    • ZonedExtra::forLocalDateTime() -> forPlainDateTime()
    • OffsetDateTime::localDateTime() -> plainDateTime()
    • OffsetDateTime::localDate() -> plainDate()
    • OffsetDateTime::localTime() -> plainTime()
    • PlainDateTime::localDate() -> plainDate()
    • PlainDateTime::localTime() -> plainTime()
    • The old methods are retained for backwards compatibility, but are
      marked as deprecated.
  • Breaking Replace output parameter fold with resolved:
    • ZonedDateTime::fold becomes ZonedDateTime::resolved
    • OffsetDateTime::fold becomes OffsetDateTime::resolved
    • PlainDateTime::fold becomes PlainDateTime::resolved
    • PlainTime::fold becomes PlainTime::resolved
  • Breaking Replace input parameter fold with disambiguate in the
    following methods:
    • ZonedDateTime::forComponents()
    • ZonedDateTime::forPlainDateTime()
    • ZonedDateTime::normalize()
    • ZonedExtra::forComponents()
    • ZonedExtra::forPlainDateTime()
  • Add 2 more zonedb databases
    • zonedb2025
      • same as zonedb, but using transition rules for 2025 and onwards
      • used with BasicZoneManager and `BasicZoneProcessor
      • larger than zonedb because it supports more zones than
        zonedb (almost all zones in the TZDB, except maybe 7 zones)
    • zonedbx2025
      • same as zonedbx, but using transition rules for 2025 and onwards
      • used with ExtendedZoneManager and `ExtendedZoneProcessor
      • smaller than zonedbx (by around 10 kB) because it only
        includes transitions after 2025