v0.6.0

A minor rather than a patch: a pair of targets the unifier used to accept is
now refused. Composer's caret treats that as breaking on 0.x, and it is —
though what those targets produced was a double that logged an argument value
neither contract declares.

• Conflicting parameter defaults across unified contracts rejected the
  target instead of quietly becoming null. Understudy::for(A::class, B::class) where both declare tag(string $name, int $weight = 5) and
  = 7 rendered = null and widened the type to int|null to make that
  legal — so an omitted argument was logged as a value neither contract
  declares, and verify(fn () => $double->tag('alpha', 5)) did not match the
  call A would have made. That is the failure renderDefault()'s own
  docblock says the unifier refuses; now it does, the way the by-reference
  conflict beside it already did. A default declared in one target and absent
  or required in another is not a conflict and keeps working. Fixes #91.
• A stub armed with -0.0 was invisible to a call made with 0.0. The
  dispatch index keyed a literal first argument by serialize(), which is not
  isomorphic to ===: -0.0 === 0.0 is true and d:-0; is not d:0;. The
  index is only consulted from the second expectation on a method, so an
  unrelated second stub changed the first one's behaviour. Fixes #92.
• DispatchIndexPropertyTest gains a second property for identity, over
  literals of every scalar type — the existing one draws int only, so no
  float, let alone a signed zero, was ever reached. The pairs a key can get
  wrong in either direction are named Examples() rather than left to the
  random phase: both signed zeros, 0 against 0.0 and '0', '' against
  null and false, 1 against true, and NAN, which matches no call
  including itself.
• The parity matrix gained ForeignCapture: somebody else's zero-argument
  capture() written inside a specification. Both analysers must report it,
  and one of them did not — understudy-psalm decided a capture by the method
  name and an empty argument list, because its issue hook is handed no resolved
  receiver, and swallowed the diagnostic. Fixed in that package (#18) and
  pinned here, which is what the matrix exists for.
• perf/README.md records that the figures were re-verified for the v0.5.0
  tag and left unchanged: three runs a side at the commit they were taken from
  and on the release candidate, with our movement inside the competitors'.