Add a client-wide default for the synchronized operation flag

[loevgaard]

Why

Call sites that always want synchronized operations currently have to repeat the flag on every call:

$client->payments()->refund(
    $paymentId,
    new RefundRequest(amount: $amount),
    synchronized: $this->api->isSynchronized(), // repeated on every operation call
);

Whether to wait for the completed transaction is almost always an integration-wide decision, not a per-call one — so it belongs on the client, not at every call site. (It deliberately does not move into the request DTOs: ?synchronized is a query flag rather than body data, the Payload DTOs map 1:1 to Quickpay's documented request bodies, and cancel() has no body DTO at all.)

What

• Client constructor gains a bool $synchronized = false parameter (appended last — BC for positional callers), exposed as isSynchronized() on ClientInterface.
• The operation methods (authorize/capture/refund/cancel) change bool $synchronized = false → ?bool $synchronized = null; null falls back to the client-wide default, a non-null argument overrides it. Behavior is unchanged for existing code.
• README + CLAUDE.md updated.

$client = new Client('YOUR_API_KEY', synchronized: true);

$client->payments()->capture($id, new CaptureRequest(1000)); // waits (client default)
$client->payments()->refund($id, new RefundRequest(250), synchronized: false); // override per call

BC note

Adding isSynchronized() to ClientInterface breaks custom implementations/decorators of that interface (if any exist). Consumers using Client directly are unaffected.

Tests

• Client default is false / constructor flag is exposed via isSynchronized().
• An operation on a synchronized: true client appends ?synchronized without a per-call flag.
• A per-call synchronized: false overrides a true client default.
• Full suite: PHPUnit (92 tests), PHPStan max, ECS, Rector dry-run, Infection all green locally (covered MSI ≈ 76% ≥ 70 gate; the mutant flipping the new constructor default is killed).

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