Schema: schema caching and rebuilding across tests #125

Part of #102. Line numbers refer to branch chore/php-8.4-dependabot (#98). Verified = reproduced, suspected = from reading the code.

• The static $executedStatements cache is keyed by SQL string and is append-only. If class B defines user with one more column than class A, the run fails with table user already exists (SqliteFileBasedSchemaStrategy.php:107-113, MysqlBasedSchemaStrategy.php:44-50, MysqlDamaBasedSchemaStrategy.php:47-53). Verified (SQLite file), suspected for MySQL.
• The SQLite file DAMA strategy runs CREATE TABLE unconditionally (SqliteFileDamaBasedSchemaStrategy.php:44-46). If the file already has tables, every test fails with table already exists. This happens e.g. after a test with #[SkipDatabaseRollback] switched to the non-DAMA strategy, or after a run with DAMA disabled. Verified (emulated).
• The SQLite file strategy unlinks the database file under an already open connection, so CREATE runs on the deleted inode (SqliteFileBasedSchemaStrategy.php:66-70). Verified.
• SQLite memory runs CREATE TABLE unconditionally, so a second fixture on the same connection fails (SqliteMemoryBasedSchemaStrategy.php:22-27). Verified.
• Nested database directories are not created, because mkdir() isn't recursive (SqliteFile*SchemaStrategy). Verified.

Possible approach: a schema-aware rebuild via DBAL's Comparator instead of the SQL-string cache, and a schema cache that survives between runs, keyed by a schema hash.