keeps_wall: invert to an is_entry mark on tiles[0]; a lone seeded pane whose attach is refused ends mux

open   by a73x

From the wall-is-the-layout branch (2026-09-02). Task 8 tried to delete Tile.keeps_wall and found the entry tile and a picker-born tile need opposite outcomes on the same refusal (entry: finish rc 1; picker: vanish, wall stands). The flag stays, documented. Follow-up: mark the entry tile instead (retry_cold is already false only there) so every other tile keeps the wall — which also fixes the edge where a lone SEEDED pane whose attach is refused ends mux with 'attach refused' instead of standing as a refused pane. Untested today.