[Bugfix] Compensate cancelled duplex open, append and resume; fix serving README drift (#7636 Issues 1, 2, 3, 21) (#7785)

Signed-off-by: chickeyton <ngton2014@gmail.com>
Signed-off-by: Yueqian Lin <70319226+linyueqian@users.noreply.github.com>
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
Co-authored-by: Yueqian Lin <70319226+linyueqian@users.noreply.github.com>

docs/design/fullduplex.md

@@ -283,8 +283,13 @@ command  handle.submit(DuplexCommand)
          -> DuplexSessionManager.dispatch: unknown_session / input_backpressure checks, then runner mailbox
 output   DuplexOrchestrator._intercept_stage_output -> runner.on_stage_output -> mailbox -> typed events
          -> output_sink (DuplexSessionEventMessage) -> DuplexOmni._route_engine_message -> handle.events()
-detach   DuplexOmni.detach_session -> touch(DETACH): engine-owned disconnect grace; expiry -> SessionExpired
-resume   DuplexOmni.resume_session(expected_lease_generation) -> lease CAS; the existing handle is re-entered
+detach   DuplexOmni.detach_session(expected_lease_generation) -> touch(DETACH): engine-owned disconnect
+         grace, refused for a lease newer than the one the caller held; expiry -> SessionExpired
+resume   DuplexOmni.resume_session(expected_lease_generation) -> lease CAS keyed by the control id (a replay
+         answers with the generation it produced); the existing handle is re-entered. A caller cancelled
+         mid-RPC observes the outcome afterwards (replaying until the engine answers) and settles a landed
+         resume: the generation goes to the resume waiting to activate, else to the connection still
+         attached, else the lease is detached again
 close    DuplexOmni.close_session -> close RPC; the manager tears the runner down, then the stage cleanup
          (abort submitted requests, release reserved ids), then SessionClosed, then the RPC result.
          SessionClosed is emitted after the cleanup attempt (in a finally), so it is also sent when the