docs(design): AT4DX binding validate/create/set and field-set-inclusion (0015/0016)

[ClayChipps]

Summary

• 0015 designs simply aep at4dx binding validate/create/set for the existing Service/Selector/Domain Application Factory binding family (UnitOfWork explicitly out of scope). Confirmed against AT4DX's actual shipped schema (not guessed) that To__c is platform-unique on all three, and BindingSObject__c/BindingSObjectAlternate__c are additionally unique on Domain — so a new duplicate-to/duplicate-domain-sobject pair of validate rules is proposed, and ambiguous-sobject-reference is error here (backed by a real, confirmed AT4DX validation rule), not warning like 0010's DomainProcessBinding version.
• 0016 designs a brand-new command family, simply aep at4dx field-set-inclusion list/validate/create/set, for SelectorConfig_FieldSetInclusion__mdt — a CMDT with no public documentation and no existing support in this repo. Its schema was pulled directly from apex-enterprise-patterns/at4dx rather than assumed. Notable finding baked into the design: FieldsetName__c is unique org-wide, not per-SObject.
• Both docs reuse the ENTITY_DEFINITION_STANDARD_OBJECTS/isCustomObjectApiName eligibility check from 0014 (PR feat(simply-aep): validate EntityDefinition eligibility for domain-process-binding SObject fields #140), proposing to extract it into a shared entityDefinitionEligibility.ts since a third CMDT now needs it.
• Docs only — no implementation in this PR. Implementation follows in separate PRs per doc, starting with 0015.

Test plan

• N/A (documentation only). Implementation PRs will carry their own test plans per each doc's Testing section.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com

https://claude.ai/code/session_014oTGUGohBiqrzETHCs25AB