refactor(objectql): move five delete-cascade bodies out of engine.ts into engine-delete-cascade.ts

Part of the engine.ts seam split: collectCascadeDeleteSet,
refuseRestrictsInsideOwnCascade, deleteRestrictedError,
referenceProbeFilter and recordReferenceCheckElevation move unedited
behind DeleteCascadeHost; cascadeReachOf moves with its only reader;
four shared helpers and CascadeDeleteSet are exported in place.

Claude-Session: https://claude.ai/code/session_01L6bhgXPvYEKGGY6sms31q2
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

content/docs/permissions/system-context.mdx

@@ -523,6 +523,7 @@ and CI refuses one that differs from what the tree produces.
 | `packages/metadata-core/src/meta-write-capability.ts#metaWriteCapabilityVerdict` | 1 | 51 |
 | `packages/metadata-core/src/meta-write-capability.ts#walledCatalogWriteVerdict` | 1 | 55b |
 | `packages/metadata-core/src/object-schema-fls.ts#isObjectSchemaMaskExempt` | 1 | 8 |
+| `packages/objectql/src/engine-delete-cascade.ts#recordReferenceCheckElevation` | 1 | 29 |
 | `packages/objectql/src/engine-driver-options.ts#buildDriverOptions` | 1 | 24 |
 | `packages/objectql/src/engine-execution-plumbing.ts#buildSession` | 1 | 63 |
 | `packages/objectql/src/engine-master-detail-related.ts#resolveOneHopRelated` | 1 | 29b |
@@ -534,7 +535,6 @@ and CI refuses one that differs from what the tree produces.
 | `packages/objectql/src/engine.ts#inheritsElevation` | 1 | 1b |
 | `packages/objectql/src/engine.ts#insert` | 3 | 21, 22 |
 | `packages/objectql/src/engine.ts#isSystem` | 1 | 64 |
-| `packages/objectql/src/engine.ts#recordReferenceCheckElevation` | 1 | 29 |
 | `packages/objectql/src/engine.ts#update` | 4 | 19, 20 |
 | `packages/observability/src/perf-timing.ts#isPerfDisclosurePrincipal` | 1 | 13 |
 | `packages/plugins/organizations/src/organizations-plugin.ts#start` | 1 | 62 |