docs(spec): regenerate objectstack-api skill references for droppedFields transitive deps (#3455)

batch.zod.ts now imports DroppedFieldsEventSchema from data-engine.zod, whose
transitive imports (kernel/execution-context, security/explain) pull those into
the objectstack-api skill's referenced-schema set. Regenerated via gen:skill-refs.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: os-zhuang

skills/objectstack-api/references/_index.md

@@ -23,9 +23,12 @@ from `node_modules` — there is no local copy in the skill bundle.
 
 - `node_modules/@objectstack/spec/src/api/contract.zod.ts` — Standard Create Request
 - `node_modules/@objectstack/spec/src/api/realtime-shared.zod.ts` — Realtime Shared Protocol
+- `node_modules/@objectstack/spec/src/data/data-engine.zod.ts` — Data Engine Protocol
 - `node_modules/@objectstack/spec/src/data/field.zod.ts` — Field Type Enum
 - `node_modules/@objectstack/spec/src/data/filter.zod.ts` — Unified Query DSL Specification
 - `node_modules/@objectstack/spec/src/data/query.zod.ts` — Sort Node
+- `node_modules/@objectstack/spec/src/kernel/execution-context.zod.ts` — Execution Context Schema
+- `node_modules/@objectstack/spec/src/security/explain.zod.ts` — [ADR-0090 D6] Access-explanation contract — `explain(principal, object,
 - `node_modules/@objectstack/spec/src/shared/expression.zod.ts` — Expression Protocol
 - `node_modules/@objectstack/spec/src/shared/http.zod.ts` — Shared HTTP Schemas
 - `node_modules/@objectstack/spec/src/shared/identifiers.zod.ts` — System Identifier Schema