fix(pty): retain backend cleanup failures

packages/pty/pty/README.md

@@ -4,10 +4,10 @@ Owner-scoped persistent PTY seam. `PtyService` registers as `ctx.pty`, mints opa
 
 ## Contract
 
-- Backends register one stable `type` and return an unpublished `PtyBackendSession`; failed or cancelled setup must clean partial resources.
+- Backends register one stable `type` and return an unpublished `PtyBackendSession`; failed or cancelled setup must clean partial resources, and a failed cleanup rejects with `PtyBackendCleanupError` so the registry can retain it across cancellation.
 - Spawn cancellation preserves the caller's exact abort reason. Service disposal and owner loss remain distinct machine-routable failures after backend setup.
 - Owner and service disposal abort unpublished setup through a service-owned signal and await backend settlement plus rollback before returning.
-- A rollback close failure rejects both the spawn and the disposing lifecycle instead of claiming quiescence.
+- A service rollback or backend-reported startup cleanup failure rejects the disposing lifecycle instead of claiming quiescence; the spawn caller still receives its exact cancellation reason.
 - `hasOwnerActivity(owner)` spans unpublished setup through final close, so lifecycle policy can fence the exact owner without a publication race.
 - A successful spawn publishes one `PtySessionId`. The optional `name` is owner-local display metadata, never authority.
 - One session accepts at most one live send operation. Reads and signals may observe it; another send fails until the operation settles.