feat: Make the model mutable

[odolbeau]

Editing an imported Card meant rebuilding every object, copying each of its arguments by hand. Model classes are no longer read-only:

$card = (new VCardDecoder())->decode($vCard)[0]->value;
$card->name->full = 'Jane Doe';
$card->emails['EMAIL-1']->address = 'jane@example.com';
$card->phones['work'] = new Phone('+33 1 23 45 67 89', contexts: [Context::WORK]);
unset($card->notes['NOTE-1']);

• readonly removed from the 30 classes of src/Model; they stay final, typed, with promoted properties.
• The encoders still validate the Card when they write it; nothing is checked on assignment.
• clone stays shallow: the README points to myclabs/deep-copy, added to suggest, for an independent copy.
• Importer: three object rebuilds become plain assignments (withGeography() removed, linkTitles(), the JSPROP fallback in patch()).
• README: new "Editing a Card" section; CHANGELOG entry under Unreleased.
• New tests/Model/EditingTest.php: decode, edit, and check the exact vCard 4.0 and JSON output.

make qa passes.

🤖 Generated with Claude Code