feat: #3586613 JSON Schema content-entity-reference

• Added well-known json schema shape + docs
• Add content-entity-reference prop shape; scope entityFields via conditions
• Suggest entity-reference prop sources from host-entity fields
• Resolve entity-reference props at render time in JsComponent
• Test entity-reference matching
• Inject entity-reference constraints into JsComponent SDC definition
• Harden entity-reference prop validation + tests
• Test entity-reference prop storage and x-allowed-* projection
• Test toSdcDefinition idempotency and updates
• Test content-entity-reference prop resolution in JsComponent

AI-Generated: Yes (Used Claude to create a plan / split / code). Reviewed every single line and I own it.

Testing instructions

• This is infra needed for META #3573831, so cannot really be tested independently. We still need to build the UI.

Closes #3586613