feat(Spec): add plain OpenAPI 3.2 fields

[DerManoMann]

Overview

This adds 3.2 fields that need nothing but a property and somewhere to emit it. Spec
attributes stay version-agnostic — one DTO carries every version's fields and the
per-version compiler decides what to emit — so each field is carried on the DTO and
emitted only by OpenApi32Compiler. A 3.0 or 3.1 document drops it silently, the way it
already drops Tag::$parent.

Along the way this settles a convention that was never written down: a mixed property
cannot use null to mean "unset", because null is a legal value for it.

Changes

• OpenApi::$self — a URI identifying the document, and the base URI for relative
  references
• Server::$name — a unique name for the server, for use in tooling
• Response::$summary — a short summary alongside the existing description
• Example::$dataValue — the example value before serialization
• Example::$serializedValue — the example value after serialization
• Security\Scheme::$deprecated — applies to every scheme type
• Security\Scheme::$oauth2MetadataUrl — the RFC 8414 authorization server metadata URL,
  emitted for oauth2 schemes only
• deprecated and oauth2MetadataUrl also reach the typed Security\Scheme\* subtypes
• The Example Object's mutual-exclusion rules are validated with a warning, matching the
  existing License url/identifier check
• Example::$value and Link::$requestBody move to Undefined::UNDEFINED, the sentinel
  every other mixed property already used, so both can now express an explicit null.
  Compiled output is otherwise unchanged, and HybridBridge passes both straight through
  instead of translating them
• The convention is written up in docs/dev/pipeline.md
• A redundant inline @var on OpenApi::$security is dropped