feat(pdo): add from-connection to wrap an existing PDO

[Chemaclass]

Adds pdo/from-connection so Phel can reuse a host framework's already-open \PDO (Symfony/Laravel + Doctrine DBAL's getNativeConnection()) instead of opening a second connection:

(def conn (pdo/from-connection php-pdo))
(-> (pdo/query conn "select * from products where id = 1") (pdo/fetch))

Behaviour (per #21):

• Returns the standard connection value, so every downstream fn (query, prepare, execute, fetch, insert, begin/commit/rollback, with-transaction, ...) works unchanged.

• Leaves attributes untouched by default - the host owns the connection's configuration. {:apply-defaults true} opts into phel-pdo's ERRMODE_EXCEPTION.

• Lifecycle: phel-pdo never closes a connection it did not open (the library exposes no close, so a wrapped handle is never closed by it).

• Tests: wrap a pre-opened in-memory SQLite \PDO and run query/fetch; run a with-transaction; verify default leaves ERRMODE as set by the host (SILENT preserved) and :apply-defaults forces ERRMODE_EXCEPTION.

• README API table + docs/recipes.md "reuse an existing (framework) connection" section + CHANGELOG updated.

Polish pass: no refactor needed - reuses the connection struct and set-attribute; no duplication.

composer test green (60 assertions).

Closes #21