Add first-class JSON columns and fix the SQLite test run

[twoixter]

Summary

JSON columns (MySQL/MariaDB JSON, PostgreSQL json/jsonb, SQLite JSON) were mapped to STRING: reads returned raw text, assigning an array stored Array, and to_json() double-encoded the document. They now become Column::JSON and hold an ActiveRecord\Json value object.

• Json implements ArrayAccess, Countable, IteratorAggregate and JsonSerializable. Nested elements are returned by reference so writes like $doc->payload['tags'][] = 'x' land in the document. Top-level writes flag the model dirty at once; nested in-place changes are detected on save() against a clean snapshot. A top-level {} survives the round trip, and reading a missing key does not alter the document.
• Column::cast() parses strings as JSON text, so existing code that assigned json_encode()'d strings keeps working, and wraps arrays, scalars and objects. cast_default() keeps expression defaults from breaking metadata loading.
• Model casts objects for JSON columns, copies a document assigned from another model or attribute, gives each new record its own copy of a JSON default, and folds document changes into is_dirty(), dirty_attributes() and attribute_is_dirty(). Table encodes documents on insert and update.
• static $json_attributes on a model opts text columns into the same handling.
• The MySQL adapter detects MariaDB JSON columns, reported as longtext, through their json_valid() check constraints.
• to_json() and to_array() nest documents; XML writes them as elements and CSV as JSON text.
• README gains a "JSON columns" section.

SQLite as the default adapter

The suite now passes with PHPAR_ADAPTER=sqlite (master had 147 errors and 2 failures):

• The SQLite adapter only claims ORDER BY/LIMIT support on UPDATE/DELETE when the build has SQLITE_ENABLE_UPDATE_DELETE_LIMIT. Debian/Ubuntu builds (CI) have it; macOS does not, so those two tests skip there.
• Connection::query() binds values with a PDO type matching the PHP type. execute() bound everything as text, and SQLite does not convert text back to a number against an expression with no affinity, so an association condition like length(title) = ? never matched. Booleans deliberately stay text so PostgreSQL keeps accepting true for integer columns.
• DatabaseTest rebuilds the SQLite schema when a previous class deleted the file. Two tests that assumed a network URL, or a database that rejects IN() with no values, handle SQLite explicitly.

Known limits

• Documents decode to associative arrays, so a nested empty object is written back as []; only the top-level {} is preserved.
• MySQL reorders object keys and normalises whitespace; the tests compare without regard to order.

Tests

• test/JsonTest.php covers the value object and casts without a database.
• MysqlJsonTest, PgsqlJsonTest and SqliteJsonTest run the same model-level cases (test/helpers/JsonModelTestCase.php) against each database in every job through a new documents table, independently of the default adapter.
• No existing test was changed except the three SQLite accommodations above.

Verified locally: 911 tests green on MySQL 5.7 and 8.4, MariaDB 11.8, PostgreSQL 18 and SQLite, with each of mysql, pgsql and sqlite as the default adapter, and phpcs clean.