v0.17.0

• Changed. opis/json-schema 2.6.2 no longer applies the object form of
  additionalProperties to members properties declares: such a member's
  failure is reported under its own properties subschema only, never twice.
• Added. PSR-15 request middleware ContractMiddleware (observe-only by default,
  strict() rejects with 400 + application/problem+json violation summary) and the CLI fixture runner
  bin/openapi-contract <contract-file> <fixtures-dir> with CI-friendly exit codes (#188).
  Runtime dependencies gained psr/http-factory and psr/http-server-middleware;
  nyholm/psr7 stays a dev dependency and a suggest — the CLI runner needs any
  PSR-7 implementation installed and says so when it is missing.
• Raised vimeo/psalm to ^7.0 (dev dependency); SchemaValueDecoder::coerce() now declares
  array<string, mixed> map values it really takes.