Record Phase 10 in the plan and CLAUDE.md

Marks Phase 10 done, adds the outcome notes — one collection filtered four
ways, trip dates rather than booking dates, seats held rather than heads, and
why the demo seeder exists at all — and adds seven Phase 10 cases to the
plan's list of tests worth writing.

8 and 9 stay deferred and stay required before go-live.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
