Active BackendHandle probe via running-process backend_sdk (SDK measurement run)

[zackees]

Downstream follow-up from the running-process broker v1 second-pass API post-mortem (zackees/running-process#412, §7.4 — the §6 measurement run).

fbuild currently has only a service-metadata/direct-fallback seam + diagnostics preview (fbuild#529/#530), no running-process code dependency. The second-pass backend_sdk now ships in running-process, so the deferred active BackendHandle probe is the natural fresh-integration measurement run.

Scope

• Add the running-process dep (features client, backend-sdk).
• Probe via BackendHandle::probe_with_service_async + a DaemonProcess identity sidecar in daemon discovery.
• Serve probes via BackendEndpointMux in the fbuild daemon accept loop.
• Validate with the conformance kit per running-process docs/INTEGRATE.md.

Acceptance (measurement target)

• One PR, < 100 consumer LOC, single-doc context (INTEGRATE.md + SDK rustdoc).

[zackees]

Closing as completed by the merged running-process broker/backend work now on main.

Evidence checked on current origin/main:

• Workspace Cargo.toml now depends on running-process = 4.3.0 with the async client feature, and crates/fbuild-daemon consumes the workspace dependency.
• crates/fbuild-cli/src/daemon_client.rs attempts AsyncBrokerSession::adopt(OwnedConnectRequest::new(...)) before direct HTTP fallback, recording broker-negotiated daemon acquisition.
• crates/fbuild-daemon/src/main.rs starts fbuild_daemon::broker::backend::spawn_backend_endpoint_if_requested(...) during daemon startup.
• crates/fbuild-daemon/src/broker/backend.rs binds the broker-provided backend endpoint, creates a DaemonProcess sidecar, answers BACKEND_HANDLE_PROBE_PAYLOAD_PROTOCOL frames via endpoint_probe_response_frame, and serves broker-framed health/daemon-info requests.
• crates/fbuild-daemon/README.md documents that the daemon answers BackendHandle identity probes and retains HTTP operation transport for this migration slice.

Verification run:

• cargo test -p fbuild-daemon broker::backend -- --nocapture passed, including replies_to_backend_probe_frames and broker health frame coverage.
• cargo test -p fbuild-cli daemon_client -- --nocapture passed, including broker negotiation/fallback tests.

No new PR is needed for this issue because the requested implementation is already present on the default branch.