MCP builds without client restarts

tassix_mcp_launcher is what .mcp.json starts: it runs the build cargo xtask mcp names in target/mcp/current.json and relays the session. When a newer build is installed it swaps between requests: the running server hands over its session (tassix/session: edited scene, scene file, undo/redo history, reviews, permission), the new one is introduced to the client as before and restores it (tassix/restore), and the client gets notifications/tools/list_changed. Not in play mode; a server that stops is restarted. The review panel's token comes from the launcher, so its link survives.
editor_core exports and imports its history (transactions with their footprints, review entries), so undo, redo and pending reviews continue in the new process.
Integration tests run the launcher with the real server: swaps with the session (a Unity scene that still saves back afterwards), deferral in play mode, restart after a crash.