Update package:intl4x for Dart and add likely_subtags support

[mosuem]

Summary

This PR updates the Dart test executor to support the latest package:intl4x and package:icu4x, adds likely_subtags support across both Native and Web platforms, and improves test configurations and CI stability.

Key Changes

1. likely_subtags Implementation:

   • Implemented likely_subtags using package:intl4x/locale.dart unified across both Dart Native and Dart Web:
     • Locale.maximize().toLanguageTag() for maximize
     • Locale.minimize().toLanguageTag() for minimize and minimizeFavorRegion
     • minimizeFavorScript is marked as unsupported, aligning with ECMAScript 402 (Intl.Locale.prototype.minimize()), ICU4C, and ICU4J.
   • Removed platform-specific files (likely_subtags_native.dart and likely_subtags_web.dart).

2. Dependencies & Versioning:

   • Updated icu4x dependency to ^2.3.1.
   • Pinned package:intl4x to git commit a3c72590b83f60c3631ece8064d7990840eed187 (dart-lang/i18n#1091), which introduces maximize(), minimize(), and canonicalize() on Locale.
   • Regenerated version tracking files via bin/set_version.dart and bin/make_runnable_by_node.dart.

3. Plural Rules & Datetime Format:

   • Adapted plural_rules.dart for the updated PluralRules.select API.
   • Fixed timezone formatting and display names handling.

4. Test Configurations (run_config.json):

   • Added likely_subtags to all Dart configurations (Native on icu77, Web on icu73–icu78).
   • Added dart_web on icu78 (Node 26.3.0).

5. CI Stability (verifier/verifier.py):

   • Capped multiprocessing in verifier.parallel_verify_data_results to min(2, mp.cpu_count()) (matching schema/schema_validator.py) to prevent runner OOM (Exit Code 137) when verifying large test plans (e.g. collation).

[gemini-code-assist]

Code Review

This pull request updates the Dart SDK environment constraint to ^3.12.0 and upgrades several dependencies in pubspec.yaml and pubspec.lock. It also attempts to change how dateStyle and timeStyle are added to the options map in datetime_format.dart. However, the introduced syntax 'dateStyle': ?dateStyle is invalid in Dart and will cause a compilation error. It is recommended to revert to the original collection-if syntax (if (dateStyle != null)) to conditionally include these keys.

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