fix(client,host): late duplicate-provider conflicts roll back wholesale and fail loud

Holes declared after two flow providers activated left the loser throwing
out of the slot flush with partial occupancy. deferRegistration gains an
onFailure channel (late failures unsubscribe, then hand over instead of
throwing through the flush); the native flow's pair rolls back wholesale
and re-raises on the global channel the boot's fail-loud handler owns.
Duplicate rows of the SAME package stay silently idempotent (the component
identity guard skips an occupied hole).

packages/client/ui-slots/src/deferred.ts

@@ -36,6 +36,12 @@ export interface DeferredRegistration {
  * @param name - target slot name.
  * @param component - the component whose ledger presence marks "registered".
  * @param register - performs the actual registration; returns its disposer.
+ * @param onFailure - owns a registration failure that fires from a LATER
+ * ledger flush (a declaration landing after two providers deferred, say):
+ * the deferral first removes its own subscription, then hands the error
+ * over instead of throwing through the flush — the callback's chance to
+ * roll back sibling deferrals and surface the conflict on a loud channel.
+ * Absent, a late failure rethrows out of the flush.
  * @returns the deferral handle (dispose in the owning effect's disposer).
  * @throws the immediate registration's failure, after removing the
  * just-installed subscription — a throwing construction leaves nothing live.
@@ -45,6 +51,7 @@ export function deferRegistration(
   name: string,
   component: unknown,
   register: () => () => void,
+  onFailure?: (error: unknown) => void,
 ): DeferredRegistration {
   let dispose: (() => void) | undefined
   const tryRegister = (): void => {
@@ -52,7 +59,15 @@ export function deferRegistration(
     if (registry.entries(name).some(e => e.component === component)) return
     dispose = register()
   }
-  const unsubscribe = registry.subscribe(name, () => { tryRegister() })
+  const unsubscribe = registry.subscribe(name, () => {
+    try {
+      tryRegister()
+    } catch (error) {
+      unsubscribe()
+      if (onFailure === undefined) throw error
+      onFailure(error)
+    }
+  })
   try {
     tryRegister()
   } catch (error) {

packages/client/ui-slots/tests/deferred.spec.ts

@@ -24,6 +24,27 @@ describe('deferRegistration', () => {
     expect(core.entries(HOLE)).toHaveLength(0)
   })
 
+  it('hands a late registration failure to onFailure after unsubscribing itself', async () => {
+    const core = new SlotCore()
+    const component = (): null => null
+    const foreign = (): null => null
+    const failures: unknown[] = []
+    // Nothing is declared yet: the deferral just subscribes and waits.
+    const register = vi.fn(() => core.register({ name: HOLE } as never, component as never))
+    deferRegistration(core, HOLE, component, register, (error) => { failures.push(error) })
+    // The declaration lands with a foreign occupant racing in first: the
+    // deferral's flush-time attempt fails, unsubscribes itself, and reports
+    // through onFailure instead of throwing out of the flush.
+    core.register({ name: 'root', children: { [HOLE]: { kind: 'single', scope: 'root' } } } as never, (() => null) as never)
+    const disposeForeign = core.register({ name: HOLE } as never, foreign as never)
+    await Promise.resolve()
+    expect(failures.map(String).join('')).toContain('already has a registration')
+    // Unsubscribed: freeing the hole must not resurrect the loser.
+    disposeForeign()
+    await Promise.resolve()
+    expect(core.entries(HOLE)).toHaveLength(0)
+  })
+
   it('drops its subscription when the immediate registration throws', async () => {
     const core = declared()
     const foreign = (): null => null