Small additive DX items: variables(), deadlineAt/acquirer, Shopsystem, memoized callback payment, SDK version in User-Agent

[loevgaard]

Part of #10 (finding 10).

Five small, independent, additive improvements:

• Payment::variables(): array — your custom variables with keys and value types exactly as sent. This is deliberately a method reading $raw, not a mapped property: while implementing I verified that Valinor's camelCaseKeys() recurses into nested arrays, so a typed $variables property would silently rename a consumer's internal_ref to internalRef (and mixed values would need allowPermissiveTypes()). The docblock explains this. Live shape verified: {} when empty, value types preserved (int/bool/nested).
• Payment::$deadlineAt (?\DateTimeImmutable) and Payment::$acquirer (?string) typed — live: ISO-8601 with Z (mapped by the existing formats) / null until authorized. Appended as optional constructor params.
• CreatePaymentRequest::$shopsystem with a new Shopsystem payload (name/version, shopsystem[name|version] on POST /payments) so a plugin/module can identify itself on the payments it creates (Quickpay shows it as metadata.shopsystem_name/_version).
• Callback::payment() is memoized — a guard followed by a handler no longer decodes and maps the body twice.
• User-Agent carries the SDK version: Setono-Quickpay-PHP/1.0.0 (+https://github.com/Setono/quickpay-php-sdk) via Composer\InstalledVersions (Client::version() is public; composer-runtime-api ^2.0 added to require — it's a virtual platform package every Composer 2 install provides).

README: reading variables back after the update example; a Shopsystem tip.

Tests: variables verbatim (snake_case + nested + types) / empty / absent / hand-built; deadline + acquirer mapping and nulls; shopsystem on the wire; memoization; UA format with a known version.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.08%. Comparing base (862f98e) to head (15f764a).

Additional details and impacted files

@@             Coverage Diff              @@
##                1.x      #19      +/-   ##
============================================
+ Coverage     99.06%   99.08%   +0.01%     
- Complexity      197      203       +6     
============================================
  Files            26       27       +1     
  Lines           535      544       +9     
============================================
+ Hits            530      539       +9     
  Misses            5        5              

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