Replace the single-file SDK with a layered PSR-4 package

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Splits the single-file SDK into a layered PSR-4 package (Unitpay\ → src/) behind a thin
Unitpay\Unitpay facade. Nothing changed on the wire: same endpoints, same flat request
format, same signature algorithm, same response shapes. What changed is the PHP surface you
call. Still php >=7.4 and ext-json, cURL still optional, no runtime dependencies added.

Architecture

Layers, with dependencies pointing strictly downward:

Layer	Contents
Unitpay	Facade and composition root: form(), fluent setters, service getters
Api/	AbstractService plus the Payment / Subscription / Payout / Reference services and PendingParams
Http/	TransportInterface port and CurlTransport adapter
Signature/	SignatureBuilder
Webhook/	WebhookVerifier and IpAllowlist
Model/, Model/Enum/	CashItem and the const-classes Nds, PaymentObject, PaymentMethod, Measure, PaymentType
Exception/	Marker interface and five typed exceptions

Model/, Model/Enum/ and Exception/ are leaves and import nothing internal. Services
never construct a transport. They receive TransportInterface from the facade, which owns
the single shared instance (it also serves the webhook IP feed), so a custom HTTP stack is
injected in one place.

Breaking changes

• UnitPay.php is gone, so installation is Composer-only. The package needs the PSR-4
  autoloader and can no longer be required as a single file.
• Classes moved. UnitPay becomes Unitpay\Unitpay (note the casing), CashItem becomes
  Unitpay\Model\CashItem, UnitpayIpAllowlist becomes Unitpay\Webhook\IpAllowlist, and
  every exception moves into Unitpay\Exception\. Exception class names and their
  InvalidArgumentException / UnexpectedValueException parents did not change, so
  existing catch blocks keep working once the imports are updated.
• api('method', [...]) is replaced by four service objects reached from the facade:
  payments(), subscriptions(), payouts() and reference(). Each method takes its
  required parameters as arguments and the rest in a trailing options array. Watch out for
  getBinInfo: it sits on payouts(), next to getSbpBankList, not on reference().
• Required parameters are enforced by the method signatures now, not by the runtime
  REQUIRED_UNITPAY_METHODS_PARAMS dictionary. A missing one is an ArgumentCountError
  that static analysis catches, rather than a UnitpayValidationException at request time.
  UnitpayUnsupportedMethodException no longer applies to outbound calls, since there is no
  method-name string left to mistype; it covers inbound webhooks only.
• The fiscal dictionaries moved to const-classes in Model\Enum and lost the redundant
  prefix: CashItem::NDS_20 becomes Nds::VAT20, CashItem::PAYMENT_OBJECT_COMMODITY
  becomes PaymentObject::COMMODITY, CashItem::PAYMENT_METHOD_PAYMENT_FULL becomes
  PaymentMethod::PAYMENT_FULL, CashItem::MEASURE_KG becomes Measure::KG, and
  UnitPay::PAYMENT_TYPE_CARD becomes PaymentType::CARD. One case is irregular:
  CashItem::NDS_NONE becomes Nds::NONE, with no VAT prefix.
• Webhook verification lives behind $unitpay->webhook(), that is
  Unitpay\Webhook\WebhookVerifier: checkHandlerRequest(), getHandlerMethod(),
  getHandlerParams(), getSuccessHandlerResponse(), getErrorHandlerResponse(),
  setAllowedIps(), addAllowedIps(), getAllowedIps() and refreshAllowedIps().
  getIp() and isAllowedIp() stay protected, so running behind a proxy means
  subclassing the verifier, not the facade.
• getSignature() is no longer public on the facade. Direct signing lives in
  Unitpay\Signature\SignatureBuilder::build($params, $secretKey, $method).
• The transport constructor argument changed from a callable to TransportInterface.
  Omit it and you still get the default CurlTransport with unchanged behavior: cURL with
  a file_get_contents() fallback, TLS verified.

Explicit decisions

No compatibility shim ships. A class_alias would restore the old class names without the
old methods and constants, so code would compile and then fail at runtime. A real compat
layer would re-create the god class this release removes, and it would force dropping
final from Unitpay\Model\CashItem. Migration is a mechanical one-time edit instead.

The deprecated payment objects stay for now. excise, gambling_bet, gambling_prize,
lottery_prize and composite were announced in v2.1.0 for removal in 3.0, but pulling
them alongside the namespace break adds migration friction for no functional gain. They are
deferred to 4.0 and remain available as PaymentObject::EXCISE and friends. They should not
be used in new code.

Unchanged, and verified as such

The signature algorithm and the {up} delimiter are untouched, as is the mandatory
PHP_INT_MAX guard against signature forgery in SignatureBuilder::build(), which two
regression tests cover. Also unchanged: constant-time comparison via hash_equals(), the
REMOTE_ADDR-only IP source (never the spoofable X-Forwarded-For), TLS verification on
the IP-feed fetch, the fail-safe allowlist refresh, the flat request format, the response
shapes, PHP >= 7.4 support and the zero-dependency policy.

Tests & QA

• The suite was ported to the new namespaces and grew from 121 to 150 tests and 318
  assertions. It now mirrors the src/ tree.
• Tests\Support\FakeTransport replaces the old callable double. It implements
  TransportInterface, records the URL and headers of every call, and replays a queue of
  canned bodies; pass false to simulate a transport failure.
• New coverage for the new seams: service getters and their memoization, shared transport
  injection, PendingParams draining.
• Tooling paths follow. composer.json moves from classmap to psr-4, lint and md
  now target src, and phpstan.neon and .php-cs-fixer.dist.php do the same.

Docs

• README and the topic pages (docs/getting-started.md, docs/api-methods.md,
  docs/receipts.md, docs/webhooks.md, docs/telemetry.md) were rewritten for the
  service API. docs/api-methods.md now groups methods by service and gives full
  signatures.
• docs/migration-v3.md is new: 8 sections plus a checklist, with exact 2.x to 3.0 rename
  tables.
• All 11 examples/*.php now use vendor/autoload.php and the namespaced API.

Verification

The CI matrix was replicated locally in Docker before pushing. PHP 7.4.33, 8.0.30, 8.1.34,
8.2.32, 8.3.32 and 8.4.23 all pass: 150 tests and 318 assertions on each, 55 files linted
clean. The quality job was replayed on 8.3, where php-cs-fixer reports 0 of 55, PHPMD is
clean, PHPStan level 6 says [OK] No errors, and composer audit finds nothing.

One caveat is worth a follow-up. PHPStan exhausts the default 128M memory_limit in a
parallel worker and needs --memory-limit=512M, so the result currently depends on the
environment's php.ini. Pinning the limit in phpstan.neon or in the composer stan
script would make it deterministic. That change is not part of this PR.

Merge note

The branch was rebased onto master, so master is an ancestor and this merges
fast-forward. Please do not squash: the six commits are ordered so that each one builds and
passes tests on its own.