fix camera recovery blocking the dora event loop

Camera::open() retried forever inside a fixed sleep, so a camera that did not
come back left the node stuck there: it stopped reading dora events and no
longer honoured Stop or AllInputsClosed.

Recovery is now a state machine over ticks. open() makes one attempt and
returns; the loop decides when to try again from a 1/2/5/10 s backoff deadline,
and nothing sleeps, so events keep being read while the camera is down. The
first attempt closes the handle; past that the device is wedged and the node
escalates to sl::Camera::reboot(), guarded to the ZED 2 and newer since it does
nothing on the ZED and ZED Mini. Reopening pins the serial so recovery gets the
same physical camera back.

Set async_grab_camera_recovery so a link hiccup returns CAMERA_REBOOTING
immediately instead of blocking the grab for ~17 s, and enable_image_validity_
check so the SDK flags frames a stalled camera hands back looking valid. Those
transient errors no longer drop the handle, since the SDK is already
reconnecting it.

An all-black frame now counts as a failed grab rather than being published, and
a camera delivering nothing usable for 5 s (30 s while the SDK reports it is
reconnecting) triggers recovery. A new camera_state output publishes
opening/streaming/recovering so a downstream node can tell "no frame yet" from
"camera lost".

ZED_RETRY_INTERVAL_SECONDS is replaced by the backoff.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7g2cBJ5e3r15ZbJ9WZn8c

dataflow.yml

@@ -11,6 +11,7 @@ nodes:
       - depth
       - point_cloud
       - camera_info
+      - camera_state
 
   - id: plot
     git: https://github.com/dora-rs/dora-hub.git