Add schema migration generation, application, upload, download, and check support
for SQLite and PostgreSQL. Introduce the cloudsync_alter_* API, pending migration
tracking, raw SQL migration operations, createTable payload support, and
block-level LWW/augment migration commands.

Update libcurl and Apple native network layers, add SQLite/PostgreSQL/cross-engine
tests, refresh examples and documentation, and remove the old begin/commit alter API.

Add focused migration unit coverage for dialect override behavior, generated migration JSON replay across simulated devices, and row payload retry after applying the missing migration.

Fix the dialect override builder so SQLite/PostgreSQL overrides no longer mutate the portable nullable flag and replacing an override clears stale defaultSql state before preview/apply.

Allow PostgreSQL declarative alters to accept non-text default arguments through an anyelement overload and stringify defaults before passing them to the shared migration builder.

Validate with SQLite migration tests and PostgreSQL schema migration coverage, including non-text defaults for integer, boolean, and json columns.