feat(carriers): start new careers at the hiring carrier's terminal

[Orinks]

Carrier slice 2: the new-career start flow uses carrier-owned home terminals.

#253 has merged; this PR targets feat/career-2.0 and is rebased on its tip a38930f8.

Changes

• Flow order: region → home city → carrier. Every item is plain menu text for screen readers.
• Picker: lists only cities where is_offerable_home_city is true and a start option offers a carrier that hires there. Alaska, British Columbia, and Yukon stay blocked until an Alaska regional exists. A region with no offerable city is left out.
• Carrier list: only carriers that hire in the picked city. Nationals appear for every lower-48 city; Prairie Link appears only within 250 air miles of its Kansas City, Omaha, or Wichita terminals. Each row ends with "Truck starts at {Carrier} {City} terminal."
• Start location: the truck starts at the hiring carrier's nearest terminal (current_city = home_terminal_city, parked_facility = "{Carrier} {City} terminal"). A Milwaukee pick with Northstar starts in Chicago at Northstar Freight Lines Chicago terminal. A start never falls back to another carrier's far-away terminal.
• First-day briefing: says orientation and truck assignment happen at the carrier terminal (owner-operators: lease orientation), with no home-time claims.
• Career reset: restarts from the old home terminal city.
• Unchanged: the Great Lakes fallback on termination and solvency (change_carrier).
• Orientation travel: when the carrier terminal is in another city than the picked home city, the company-driver briefing adds "The carrier covered your travel from {Home city} and a hotel for orientation." Owner-operators and hires whose terminal is in their home city don't hear it. The picked city is saved as home_city (the integrity export fixture and blocker row include it), and a ROADMAP row for the carrier home-time slice keeps the terminal city from silently becoming the driver's home.
• CHANGELOG says the picker now enforces the home-base rule and adds a start-flow bullet. ROADMAP slice 2 rows are checked off.

Tests

• ff-core: test_a_start_puts_the_truck_at_the_hiring_carriers_terminal, test_a_regional_start_out_of_range_never_takes_a_far_away_home, test_start_options_for_a_city_are_only_carriers_that_hire_there
• app: test_home_picker_offers_only_cities_where_a_carrier_hires (Healy, Anchorage, and Whitehorse excluded; Chicago and Milwaukee included), test_milwaukee_pick_with_northstar_starts_at_the_chicago_terminal, test_carrier_list_shows_only_carriers_that_hire_in_the_home_city (Wichita/Hutchinson vs Boston), test_prairie_link_row_near_wichita_starts_at_the_wichita_terminal, test_carrier_rows_say_where_the_truck_starts, test_owner_operator_briefing_names_the_lease_terminal, test_every_offered_home_city_has_a_carrier_that_hires_there, test_a_hire_away_from_the_terminal_hears_the_carrier_covered_travel, test_a_hire_in_the_terminal_city_hears_no_travel_line, test_an_owner_operator_hears_no_travel_line
• Start-default tests updated for the new order: main-menu new-career walk, app smoke flows, online-offer picker, legacy same-name guard, first-day briefing rows, and the dispatch playtest scenario.