Add dbtool fold: emit self-contained baseline from registered migrations

[christianparpart]

Adds a new offline dbtool fold subcommand that walks all registered migrations and emits a single self-contained baseline — either a .cpp migration plugin or a .sql script — that reproduces the post-migration schema and schema_migrations rows from an empty database. Useful for collapsing a long migration history into a fast-to-apply starting point, or for shipping a snapshot baseline alongside a release.

The command is purely offline: it never opens a DB connection, never queries a live schema. It loads plugins, walks each migration's Up() plan in timestamp order, folds the cumulative effect into a per-table view + chronological data steps, and emits via the existing ToSql() formatter path so each dialect's CREATE TABLE / INSERT codegen stays the single source of truth.

Changes

• New MigrationManager::FoldRegisteredMigrations(formatter, upToInclusive) primitive — pure plan-walk, returns PlanFoldingResult (per-table state, creation order, indexes, chronological data steps, in-range releases). Used by the new module and available to any future caller.
• New Lightweight::MigrationFold library module under src/Lightweight/MigrationFold/:
  • Folder — thin facade plus ResolveUpTo() which accepts an empty string (latest registered release), a numeric timestamp, or a release version string.
  • SqlEmitter — emits a flat dialect-specific .sql script, including a CREATE TABLE schema_migrations and a stamping INSERT per folded timestamp so a freshly-loaded DB looks identical to a real apply-all run.
  • CppEmitter — emits a .cpp baseline plugin wrapped in LIGHTWEIGHT_SQL_MIGRATION, with optional --emit-cmake and --max-lines-per-file for splitting very large baselines across multiple files.
• Shared CodeGen/SplitFileWriter helper that bin-packs blocks within a per-file line budget; used by CppEmitter and intentionally factored out for reuse.
• dbtool fold --output FILE [--up-to X] [--dialect D] [--emit-cmake] [--plugin-name N] [--max-lines-per-file N] — output format is picked from the file extension. .sql requires --dialect (sqlite, postgres, mssql, mysql); .cpp is dialect-agnostic. Dispatched before SetupConnectionString since fold never touches a DB; uses a connection-less GetMigrationManagerOffline variant.
• Unit tests: 10 fold cases (create + altercolumn, drop-table cleanup, chronological ordering, --up-to truncation, RawSql passthrough, column rename FK propagation, release-range filtering, ResolveUpTo parsing) + 4 SplitFileWriter cases + 2 emitter round-trip cases. Green against sqlite3, mssql2022, and postgres.

[Yaraslaut]

Thanks, left few small comments, mostly nitpicks