refactor(plugin-security): move SecurityPlugin's controlled-by-parent write gate into security-plugin-cbp-write-gate.ts (#25223 cut 3)

content/docs/permissions/system-context.mdx

@@ -108,7 +108,7 @@ that silently does not happen.
 | 8 | Metadata-plane schema masking exempt (ADR-0106 D4) | metadata-core | Get: unmasked object schema. Note: the exemption is a **caller** property — it short-circuits before the security service is consulted | `packages/metadata-core/src/object-schema-fls.ts#isObjectSchemaMaskExempt` |
 | 8b | Authoring-trail reads admitted | rest, runtime | Get: a metadata item's version history, audit rows and stored-version diff (`GET /meta/:type/:name/history`, `/audit`, `/diff`) without `studio.access` or `manage_metadata`, and a package's commit timeline (`GET /packages/:id/commits`) without `studio.access`. Note: narrower than the schema-mask exemption the draft switches ask (`isObjectSchemaMaskExempt`): neither door admits `setup.access`, which organization admins hold | `packages/rest/src/rest-server.ts#mayReadAuthoringTrail`, `packages/runtime/src/domains/packages.ts#requireCommitTrailReadCapability` |
 | 9 | `explain()` may target a principal other than the caller | plugin-security | Get: no `manage_users` / delegated-admin check | `packages/plugins/plugin-security/src/security-plugin.ts#explainAccessForCaller` |
-| 9b | The master-detail write check the `security` service serves (`checkControlledByParentWrite`, ADR-0055) answers `allow` | plugin-security | Get: `allow` for any record, with no master judged — the write path's own first exit, so a gate composing it admits exactly what the record's own system-context update would. Lose: nothing a caller reaches today: its two consumers, the `sys_attachment` and `sys_comment` parent gates, return before they ask for a system session | `packages/plugins/plugin-security/src/security-plugin.ts#checkControlledByParentWrite` |
+| 9b | The master-detail write check the `security` service serves (`checkControlledByParentWrite`, ADR-0055) answers `allow` | plugin-security | Get: `allow` for any record, with no master judged — the write path's own first exit, so a gate composing it admits exactly what the record's own system-context update would. Lose: nothing a caller reaches today: its two consumers, the `sys_attachment` and `sys_comment` parent gates, return before they ask for a system session | `packages/plugins/plugin-security/src/security-plugin-cbp-write-gate.ts#checkControlledByParentWrite` |
 | 10 | Anonymous-deny treats the caller as authenticated | core | Get: passes the 401 seam with no `userId` | `packages/core/src/security/anonymous-deny.ts#shouldDenyAnonymous` |
 | 12 | Session-resolution middleware skipped | plugin-auth | Get: no session lookup attempted | `packages/plugins/plugin-auth/src/auth-plugin.ts#start` |
 | 13 | Per-request performance timings disclosed | observability | Get: timing headers a normal caller cannot pull | `packages/observability/src/perf-timing.ts#isPerfDisclosurePrincipal` |