[Feature]: [P3] Authored base floor for elevator schedules (Set as base)

Parked from the #464 fold-in build (spec ux-elevator-schedule-dialog-2026-07-17.md §17). The floors grid marks the DERIVED base floor (lowest served lobby, else the shaft bottom) with the ◎ glyph, but spec §4.2's Set as base affordance is unbuildable without an authored base field on the schedule model, and adding one is a save-format conversation (SAVE_VERSION bump + TDT mapping: the 1994 save may carry a related byte; check the TDT spec's elevator header when the Wine harness phase runs).

Scope when picked up: the model field + coercion, dispatch's unhomed-car fallback reading it, the grid's Set as base press, serialization + migration, and the TDT parity check.

Review lane: /gds-code-review.

Start with ux-elevator-schedule-dialog-2026-07-17.md, especially sections 4.2 and 17, then trace the elevator schedule model, dispatch fallback, floors grid, and save migration paths. During the Wine harness phase, inspect the TDT elevator header for parity. Done means the authored base is editable, used for unhomed-car fallback, serialized and migrated with the SAVE_VERSION change, and covered by the TDT parity check.