refactor(client): deferGroupRegistration owns the multi-hole flow semantics

The construction-rollback + late-conflict-rollback + loud-rethrow block was
about to be a verbatim clone across the two flow packages; ui-slots now owns
it as deferGroupRegistration (one occupant, several holes, as a unit), with
direct specs for all three arms, and the native flow consumes it.

packages/host/directory-picker-native/src/client/index.ts

@@ -7,7 +7,7 @@
  * both sides of the native interaction with one cordis.yml row; no client
  * code branches on a capability kind.
  */
-import { deferRegistration } from '@deepseek-ai/dsh-client-ui-slots'
+import { deferGroupRegistration } from '@deepseek-ai/dsh-client-ui-slots'
 import type { ClientContext } from '@deepseek-ai/dsh-client-runtime/client'
 // Type-only: pulls the SlotMap merge declaring the directory-flow holes.
 import type {} from '@deepseek-ai/dsh-client-ui-workspace/client'
@@ -27,27 +27,15 @@ export const inject = ['slots', 'workspaces']
 export function apply(ctx: ClientContext): void {
   const injected = (): NativeFlowInjected => ({ pick: () => ctx.workspaces.pickDirectory() })
   ctx.effect(() => {
-    // Constructing the pair can throw halfway (a declared hole already
-    // occupied registers synchronously): roll the earlier deferral back so
-    // no live subscription outlives the failed fiber. A conflict surfacing
-    // from a LATER ledger flush (holes declared after two flow providers
-    // activated) rolls the whole pair back the same way and re-raises on
-    // the global channel the boot's fail-loud handler owns — never a throw
-    // through the slot flush, never partial occupancy from this package.
-    const deferred: ReturnType<typeof deferRegistration>[] = []
-    const lateConflict = (error: unknown): void => {
-      for (const entry of deferred) entry.dispose()
-      queueMicrotask(() => { throw error instanceof Error ? error : new Error(String(error)) })
-    }
-    try {
-      deferred.push(deferRegistration(ctx.slots, 'conversation.hero.workspace.directoryFlow', NativeDirectoryFlow, () =>
-        ctx.slots.register({ name: 'conversation.hero.workspace.directoryFlow', inject: injected }, NativeDirectoryFlow), lateConflict))
-      deferred.push(deferRegistration(ctx.slots, 'sidebar.workspaces.directoryFlow', NativeDirectoryFlow, () =>
-        ctx.slots.register({ name: 'sidebar.workspaces.directoryFlow', inject: injected }, NativeDirectoryFlow), lateConflict))
-    } catch (error) {
-      for (const entry of deferred) entry.dispose()
-      throw error
-    }
-    return () => { for (const entry of deferred) entry.dispose() }
+    // One occupant, both holes, as a unit: construction or late conflicts
+    // (holes declared after rival providers activated) roll the whole pair
+    // back and fail loud — semantics owned by deferGroupRegistration.
+    const group = deferGroupRegistration(
+      ctx.slots,
+      ['conversation.hero.workspace.directoryFlow', 'sidebar.workspaces.directoryFlow'] as const,
+      NativeDirectoryFlow,
+      name => ctx.slots.register({ name, inject: injected }, NativeDirectoryFlow),
+    )
+    return () => { group.dispose() }
   }, 'directory-picker-native: flow registrations')
 }