feat(zenoh): one session for ZenohConnector and its Ros2Connector (design 053 s11)

[lxsaah]

Stage s11 of docs/design/053-implementation-plan.md: one session for both schemes (053 §4.7). Meets criterion 8.

What it adds

let zenoh = ZenohConnector::new("tcp/192.168.10.5:7447");
let ros2 = zenoh.ros2(Ros2Node::new("cell4_gateway")).register::<Temperature>();
builder.with_connector(zenoh).with_connector(ros2);   // either order

• ZenohConnector::ros2(&self, node): a Ros2Connector on the same session. Ros2Connector::new still opens a session of its own.
• Shared (src/shared.rs, std only) holds the endpoint and config, one build-time slot per view, and a flag for the session task.
  • Each connector's build() validates exactly as before, then leaves its parts in its slot.
  • The first build() returns the session task; the other returns no futures.
  • The task takes every part on its first poll, opens the session, and serves both views in one future (tokio::join!). AimDbBuilder runs every build() before it polls anything, so both parts are there in time.
• Code split: the code after open() moved into ZenohParts::serve and Ros2Parts::serve. Behaviour is unchanged, and the s08–s10 suites pass as they were.
• A .ros2(..) view that's never registered is logged once and skipped; the Zenoh side runs normally.
• Two .ros2(..) views on one session both claim the ros2 scheme, so core's existing owned-scheme check refuses them. One ROS node per session, as 053 §4.7 says.
• with_zenoh_config on either view configures the shared session.

Tests (tests/shared_session.rs)

A TCP proxy in front of an in-process router counts every connection the database opens.

Test	Asserts
The gateway (053 §4.8), Zenoh connector registered first	a zenoh://cell4/probe/temperature put comes out as a ROS sample on 7/cell4/temperature/… with its 33-byte attachment; 1 connection
The same, ROS view registered first	same result; 1 connection
Control: ZenohConnector plus a separate Ros2Connector::new	2 connections, so the proxy really counts
A never-registered .ros2(..) view	zenoh:// publishing still works
Two .ros2(..) views registered	build() refuses them

Design change

053 §4.7 said Shared would hold core's session::OneShot. A flag in Shared does the same job, and OneShot sits behind core's connector-session feature, which this crate doesn't need. 053 §4.7 now says so and cites the test.

Checks

The full matrix runs in CI. Locally, as a quick signal: the library tests (22), shared_session (5), native_session (7), ros2_inbound (3), ros2_outbound (5), the embedded cargo check, and cargo fmt --all -- --check.

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