v0.1.0

• Initial development: the UnderstudyPHPUnitIntegration trait —
  verify-after-success via assertPostConditions(), unconditional reset in an
  #[After] hook, #[Before] guard over leaked contexts, optional
  project-wide strict stubs through understudyStrictStubs(). PHPUnit
  11.5/12/13; Pest works through uses().
• Changed: parent::assertPostConditions() now runs before verification, so a
  base class's own post-conditions are no longer skipped when an expectation
  is unmet.
• composer build now runs the integration suite as well, and CI pins each
  supported PHPUnit major in its own matrix job.
• Added @psalm-require-extends TestCase to the trait.
• An integration fixture pins that a test whose only check is an understudy
  expectation is not reported risky by PHPUnit's strictness about tests that
  assert nothing.