Migrate from SQLite.swift to PureSwift/SQLite

[colemancda]

Summary

Replaces the stephencelis/SQLite.swift dependency with PureSwift/SQLite.

• Package.swift — drop SQLite.swift + CSQLite (and the Android TARGET_OS_ANDROID branching) for a single PureSwift/SQLite dependency, which wraps the system SQLite3 on Apple platforms and links an embedded SQLite (via swift-sqlcipher) everywhere else. No more per-platform product conditions.
• AttributeValue.swift — rewrite binding conversion for the closed enum Binding instead of the Any-based protocol with as? casts; add strict textValue/blobValue accessors so .string/.data decode only from their real storage classes.
• CustomFunction.swift — replace the hand-rolled sqlite3_create_function_v2 C-API registration (and its SQLite3/CSQLite imports) with Connection.createFunction.
• Database.swift / ModelData.swift / Relationship.swift / Predicate.swift — update call sites (Connection(path:), .binding on positional values, scalar(...)?.integer, row[...]?.textValue) and drop the now-redundant custom rowDictionaries() (provided by PreparedStatement).
• EntityDescription.swift / ViewContext.swift — mutating column builder, read-only Connection init, and the borrowing/consuming ownership annotations required by the noncopyable Connection.
• Tests — update CustomFunctionTests.swift's direct SQLite usage to the new API.

Behavior is preserved — the migration is API-only.

Test plan

• swift test — all 60 tests pass, including custom-function and app-managed R*Tree prefilter tests.
• BENCHMARK=1 swift test --filter DistancePerformanceBenchmarks — both distance benchmarks pass.