Routing and browser-session fixes

Fixed

• SimBrief generation runs Route Finder when no route was supplied, after airport/date/runway updates finish, and waits for the routing result. Explicit routes are preserved. Empty or direct-only automatic results stop for review before Generate instead of silently producing DCT.
• Each active dispatcher execution lazily opens one Browserbase session and Playwright connection for its SimBrief operations. Parallel browser tools serialize on the owning thread. The session is released on completion, error or an approval pause; separate workflows remain isolated. Local chat and the live E2E runner use the same lifecycle.

Local regression tests cover session reuse, isolation, cleanup, approval pauses and routing. Live Browserbase acceptance remains pending: its API rejects new sessions after the free billable-minute allowance was exhausted. This release does not claim new live acceptance.