Add FLEx-compatible per-writing-system collation to FwLite

[hahn-kev]

Summary

• Store per-writing-system collation imported from FLEx (IcuCollationRules, SystemCollationLocale) on WritingSystem, with EF migration and import-only sync from fwdata
• Wire collation into LcmCrdt headword sort and SQLite per-WS collations via IWritingSystemCollatorProvider
• Use managed ICU4N collators in LcmCrdt (not icu.net) so custom sort works on Android/MAUI; FwData bridge still uses icu.net
• Seed the example project's German vernacular with &b < a &B < A demo ICU rules for manual testing

Test plan

• dotnet test backend/FwLite/LcmCrdt.Tests --filter "FullyQualifiedName~WritingSystemCollatorProvider|FullyQualifiedName~CustomCollationSorting|FullyQualifiedName~CanCreateExampleProject"
• dotnet test backend/FwLite/MiniLcm.Tests --filter "FullyQualifiedName~WritingSystemSync"
• Create a new example project on Android — headwords should sort Banane, Beere, Apfel, ... without crash
• Import an fwdata project with custom collation on Windows and verify headword order matches FLEx for simple tailoring rules

Notes

• ICU4N is ICU 60 (alpha NuGet); icu.net/FLEx uses ICU 72 — locale-specific ordering (e.g. Czech ch) may differ slightly from FLEx
• ICU4N.Resources adds ~18MB; consider SatelliteResourceLanguages on FwLiteMaui in a follow-up if APK size is a concern

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• Commit unit tests in branch feature/writing-system-collation

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