feat(db): add SQLite migration SQL generation

[Fhatu12]

Problem

SQLite is accepted by the migration CLI, but makemigrations --dialect sqlite had no SQL generator behind it.

Implementation

• Added a SQLite generator for the existing portable migration operations: create/drop table, add/drop column, create/drop index, and down migrations.
• Mapped portable schema types to SQLite affinity-appropriate DDL and quoted identifiers with escaped embedded double quotes.
• Added explicit diagnostics for unsupported SQLite ADD COLUMN forms before migration files are written.
• Routed makemigrations --dialect sqlite through the new generator while preserving MySQL as the default.
• Made diff ordering deterministic for stable migration output.

Executable proof

Generated SQLite migrations are executed against temporary in-memory SQLite databases in vix_db_sql_generator_tests; the tests assert table/column/index metadata, unique-index enforcement, drop behavior, and down-migration recovery for reversible operations.

Validation

• Rootless CMake/Ninja: CMake 4.4.2 and Ninja 1.13.0 from /tmp/vix-cmake-venv; no sudo/global install.
• Rootless deps: extracted Ubuntu packages under /tmp/vix-sqlite-dev.1I4RDT/extract/usr for SQLite, nlohmann_json, fmt, and spdlog.
• Configure: cmake -S . -B /tmp/vix-db-pr2-build.Z1AZof -G Ninja -DCMAKE_BUILD_TYPE=Debug -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -DVIX_DB_BUILD_TESTS=ON -DVIX_DB_BUILD_EXAMPLES=OFF -DVIX_DB_BUILD_TOOLS=ON -DVIX_DB_USE_MYSQL=OFF -DVIX_DB_USE_SQLITE=ON -DVIX_DB_REQUIRE_SQLITE=ON ...
• Build: cmake --build /tmp/vix-db-pr2-build.Z1AZof --parallel passed; built libvix_db.a, vix_db_sql_generator_tests, and vix_db_migrator.
• CTest: ctest --test-dir /tmp/vix-db-pr2-build.Z1AZof --output-on-failure passed 10/10, including vix_db_sql_generator_tests.
• Direct test: /tmp/vix-db-pr2-build.Z1AZof/vix_db_sql_generator_tests -> vix_db_sql_generator_tests passed.
• Install: cmake --install /tmp/vix-db-pr2-build.Z1AZof --prefix /tmp/vix-db-pr2-install.E5tKsS passed. Installed include/vix/db/mig/sql/MySqlGenerator.hpp, include/vix/db/mig/sql/SQLiteGenerator.hpp, lib/libvix_db.a, and libexec/vix/vix_db_migrator.
• External consumer: compiled and ran a tiny out-of-tree program against the installed SQLiteGenerator.hpp and libvix_db.a.
• Deterministic ordering: repeated the same schema diff + SQLite generation 20 times and confirmed identical SQL order.
• Static analysis: cppcheck and clang-tidy are not installed locally; no new tooling added for this pass.

Configure also confirms SQLite requested/detected ON and MySQL requested/detected OFF. The only configure warning is an upstream CMake author warning that SQLite::SQLite3 is deprecated in favor of SQLite3::SQLite3.

Compatibility

Existing MySQL generation is covered by a regression golden test inside vix_db_sql_generator_tests; MySQL remains the default dialect and no live MySQL server is required.

AI disclosure

OpenAI Codex assisted with codebase analysis, implementation, and validation. I reviewed the changes and take responsibility for the contribution.

Closes vixcpp/vix#460

Notes

This is module-first for vixcpp/db; the umbrella submodule pointer should be updated after this module commit is available upstream.