Startup profiling for the cross-session messaging inbox

What

Session startup now records named timing marks around the setup of the cross-session inbox, a local Unix-domain-socket (a file-based connection used for communication between processes on the same machine) channel that lets separate Claude Code sessions message each other.

• New timing marks bracket the dynamic import and startup call: setup_uds_start, setup_uds_imported/setup_uds_import, and setup_uds_end, corresponding to phases named setup_uds_messaging, setup_uds_dir, setup_uds_listen, and setup_uds_publish.
• A new profileStartup option is passed into the inbox startup call, which gates three more marks inside it: uds_inbox_dir_ready, uds_inbox_listening, and uds_inbox_key_published.
• If the feature is gated off, startup now logs an explicit message: "cross-session messaging gate off (will late-bind if a GrowthBook refresh enables it)".

Why

This gives clearer visibility into where time goes during cross-session messaging startup, and makes it obvious in logs when the feature is intentionally skipped versus failing.