Add custom SQL function support

[colemancda]

Summary

Adds support for registering custom scalar SQL functions with a SQLite-backed store and invoking them from predicates and sort descriptors, plus a raw-SQL escape hatch for app-managed virtual tables (e.g. RTree). The motivating use case is efficient GPS distance filtering/sorting — an app registers a distance function and, if it wants spatial indexing, sets up its own RTree table via execute — but nothing here is geo-specific.

Changes

• register(function:) on SQLiteDatabase and SQLiteViewContext — bridges a DatabaseFunction (from CoreModel) to Connection.createFunction, converting between AttributeValue and SQLite Binding at the boundary and invoking the registered closure inside the SQLite callback. Registration is per-connection, so the same function must be registered on both a database and its paired view context.
• SQLiteDatabase.execute(_:_:) — runs raw SQL against the underlying connection so apps can CREATE VIRTUAL TABLE ... USING rtree(...) and keep it in sync themselves (e.g. via triggers). The library never creates, syncs, or knows about any virtual table itself.
• Predicate.swift — compiles .function expressions to real SQL function calls (name(args...)) in WHERE clauses, with arguments compiled recursively (columns, constants, or nested calls).
• Database.swift — the ORDER BY builder emits .function sort terms the same way, appending bindings in positional order.

Testing

New CustomFunctionTests register a Haversine distance function (defined in the test, not the library) and verify:

• radius filtering via a .function predicate against an independently computed brute-force oracle,
• distance sorting via a .function sort term,
• combined filter + sort + fetchLimit,
• the same function registered on a paired SQLiteViewContext,
• unrelated (non-function) entities/queries are unaffected.

Full suite passes locally (56 tests).

Dependency note

This depends on the new DatabaseFunction / Expression.function / SortDescriptor.term API in CoreModel PR #22 (feature/functions branch), which is not yet in a released tag. CI will not pass until that PR merges and a new CoreModel release is tagged, at which point Package.swift should be bumped to that version.