Refactor library to PSR-4, add full unit-test coverage, and CI

[Joel-James]

Summary

Ships v2.0.0 of the library. The pre-refactor codebase had grown around a
static Api singleton and a kitchen-sink Service base class that mixed
HTTP, option storage, caching, and throttling. This PR splits those
responsibilities into focused collaborators, lays the file tree out as
PSR-4, and adds the test + CI infrastructure that has been missing.

Library refactor (v1 → v2)

• PSR-4 layout under DuckDev\Freemius\… — class-*.php filename
  prefixes are gone; each class lives in a file matching its name.
• No more singletons. Api::get_instance() /
  Api::get_auth_instance() are replaced by an ApiFactory that hands
  out fresh clients per call. This fixes a latent credential-leak bug
  where two callers with different credentials could share state.
• API layer split into Client (unsigned HTTP), SignedClient
  (auth headers), and RequestSigner (pure HMAC signing, no WP coupling).
• Persistence and caching extracted out of the old Service base
  class into ActivationRepository (option-backed) and TransientCache
  (per-plugin scoped cache + throttle), both behind interfaces so they
  can be swapped in tests.
• Activation data hardened into an immutable Activation value
  object with with() / with_scrubbed_license() mutators. Previously
  every service reached into the same loose string keys — a typo in any
  of them silently broke licensing.
• Plugin, ApiKeys, SiteIdentity introduced as small focused
  collaborators. Hook registration moved out of constructors into
  boot() so wiring the container has no side effects.

Tests

• 109 unit tests / 210 assertions covering every class under
  src/.
• PHPUnit 9.6 + Brain\Monkey for WordPress function mocking; minimal
  WP_Error stub in tests/Stubs/.
• Strict mode (failOnRisky, deprecations-as-exceptions).

CI / tooling

• .github/workflows/tests.yml — PHPUnit matrix on PHP 7.4, 8.0,
  8.1, 8.2, 8.3.
• .github/workflows/phpcs.yml — WPCS lint with PR annotations
  via cs2pr.
• .github/workflows/release.yml — auto-generates a GitHub Release
  from commits when a v* tag is pushed; tags containing - (e.g.
  v2.0.0-beta1) are marked as pre-releases.
• phpcs.xml.dist — WPCS ruleset with PSR-4 exemptions
  (WordPress.Files.FileName, ValidVariableName) and per-file
  exclusions for {@inheritDoc} interface methods and the HMAC
  base64_encode call.
• composer.json — bumped to 2.0.0, PSR-4 autoload + autoload-dev,
  dev deps for PHPUnit/Brain\Monkey/WPCS/PHPCompatibility, scripts
  (composer test, composer phpcs).
• README updated with the new namespace, factory wiring, and quickstart.

Breaking changes

This is a major version bump. Host plugins need to update their
integration:

• Namespaces moved (DuckDev\Freemius\Services\Service →
  DuckDev\Freemius\Services\AbstractService, etc.).
• Freemius::get_instance() keeps the same call signature but the
  collaborator graph it composes is entirely new.
• Direct callers of the old Api singletons must move to
  ApiFactory::make_public() / make_signed() /
  make_for_plugin() / make_for_install().
• Activation arrays are now wrapped in the Activation value object;
  reach for ->install_id(), ->license_key(), etc. instead of
  array keys.

Test plan

• composer test — 109/109 tests pass locally on PHP 8.x
• composer phpcs — 0 errors, 0 warnings
• CI matrix green across PHP 7.4–8.3
• Smoke-test integration in a host plugin against the
  activate.json / deactivate.json / updates/latest.json
  endpoints
• Tag v2.0.0 after merge and confirm the release workflow
  publishes notes