v0.3.0

A minor rather than a patch: two methods of the trait become @internal, and
roave/backward-compatibility-check reads that as the contract narrowing it is
— which Composer's caret already treats as breaking on 0.x.

• Allows rasuvaeff/understudy ^0.8 || ^0.9. A bridge, not a widening for
  its own sake: the engine's 0.9 is the 1.0 candidate — every contract decision
  of the 1.0 review lands there, and 1.0 follows once it has been driven by real
  packages — and a project taking it must be able to keep this adapter without a
  window in which composer require silently installs the 0.8 engine beside
  it. The ^0.8 term is dropped in the release that follows the engine's.
• The two hook methods of the trait, understudyPrepareContext() and
  understudyResetContext(), are @internal. They are protected because
  PHPUnit discovers hooks by attribute and a trait cannot hide them from the
  class using it, not as override points: a subclass that overrode one would
  silently lose the guard or the reset. understudyStrictStubs() stays the
  one documented override, and assertPostConditions() stays public API
  because the README's explicit-composition recipe calls it through an alias.
• Both READMEs stop dating lean() to «understudy 0.4+»: with a floor of
  ^0.8 every engine this adapter installs beside has it. llms.txt carries
  the family's # rasuvaeff/… heading.