rules: expand request/response-aware compatibility catalog

[webdevsamran]

Goal

Grow the breaking-change rule catalog beyond the current set, keeping the request-vs-response direction model.

Candidate rules

• Response enum value removal vs addition asymmetry
• Nested property requiredness flips inside arrays of objects
• Discriminator mapping changes on polymorphic schemas
• Content-type negotiation narrowing (e.g. dropping application/xml)
• Default value introduction changing client-visible behavior

Acceptance criteria

• Each rule has: id, severity default, docs entry in docs/rule-catalog.md, unit tests covering both directions (request/response), and a fixture demonstrating it.