[migration/ilib-js-to-dart] Convert ILibDateFmt, ILibDate, and Calendar systems to pure Dart

[gouniLee]

Checklist

The following lists affects Pub Points on pub.dev when the package is published.

• Passed the all tests.
• Verified that the example app works.
• Executed the dart format command.
• Added the API description is added if necessary.

note. The code for migrating flutter_ilib to Dart will be merged into the migration/ilib-js-to-dart branch, and once all code has been fully converted, it will be merged into the develop branch at once.

Description

Remove JavaScript interop (flutter_js) from ILibDateFmt and ILibDate, replacing
with pure Dart implementations. Implement 9 calendar systems, timezone/DST calculation,
and astronomical computations entirely in Dart using JSON locale data.

Changes

Date Formatting (lib/ilib_datefmt.dart)

• Rewrite ILibDateFmt to read ilib.data.dateformats and ilib.data.sysres directly
• Template-based rendering for all date/time tokens (yyyy, MM, dd, hh, mm, ss, etc.)
• Meridiem and month/weekday name translation from sysres locale data
• Cross-calendar conversion via Julian Day when formatter calendar differs from input
• Raw value preservation when calendars match (avoids normalization, matches JS behavior)

Date Options (lib/ilib_date.dart)

• Add _toCalendarDate() that delegates to the appropriate calendar date class
• Calendar-aware getDayOfWeek(), getWeekOfYear(), getEra() via lazy _getCalendarDate()

Timezone (lib/ilib_timezone.dart)

• Pure Dart DST calculation from ilib.data.zoneinfo JSON
• UTC offset, DST offset, and timezone abbreviation resolution

Calendar Systems (lib/calendar/)

New implementations for 9 calendars, each with Date + RataDie + Cal classes:

• Gregorian — standard civil calendar
• ThaiSolar — Gregorian + 543 year offset (Buddhist Era)
• Julian — pre-Gregorian civil calendar
• Islamic — Hijri lunar calendar (civil/tabular)
• Hebrew — lunisolar calendar with complex leap-year cycle
• Ethiopic — 13-month calendar (Ge'ez)
• Coptic — 13-month calendar (Egyptian Christian)
• Persian Astronomical — equinox-based via ILibAstro calculations
• Persian Algorithmic — 2820-year leap cycle formula

Astronomical Calculations (lib/calendar/ilib_astro.dart)

• Spring equinox computation for Persian astronomical calendar
• Delta-T, nutation, and solar longitude calculations
• Data sourced from ilib.data.astro JSON coefficients

Infrastructure (lib/ilib_init.dart)

• Asset manifest pre-check to skip unnecessary file load attempts
• Enhanced locale data merging for dateformats/sysres/zoneinfo keys

Locale Data

• Add 218 JSON locale data files for dateformats, sysres, and zoneinfo
• Regenerated from iLib v14.21.0 (CLDR 46.0)

Design Decisions

1. Rata Die as interchange format: All cross-calendar conversions go through
   Date → RD → Julian Day → target calendar RD → Date
2. No normalization on same-calendar formatting: When source date and formatter
   use the same calendar, raw year/month/day values are preserved (JS GregorianDate
   does not normalize; Dart would — so we skip conversion)
3. Lazy calendar computation: _getCalendarDate() only invoked for
   calendar-dependent tokens (day-of-week, week-of-year)

Tests

• 90+ locale-specific DateFmt tests converted from JS interop to ILibLoader
• New calendar unit tests: Gregorian, ThaiSolar, Julian, Islamic, Hebrew,
  Ethiopic, Coptic, Persian (8 test files for RataDie, 8 for CalendarDate)
• Timezone test suite converted from iLib JS test data

Test Plan

• flutter test test/datefmt/ — date formatting across 90+ locales
• flutter test test/calendar/ — calendar calculations
• flutter test test/timezone/ — timezone/DST resolution
• flutter analyze — static analysis clean