Multi-cabin search: --cabin economy,business (work-7ecx)

[ak2k]

Summary

• Extends --cabin to accept a CSV (economy,business); fans out N parallel single-cabin queries on both Matrix and gflight; client-side joins itineraries on (flight-number, date) per slice; renders one $ column per cabin with — for non-overlap. New --sort <cabin> picks the sort column (default: first cabin).
• Domain stays single-cabin — multi-cabin is purely orchestration, so wire.py / links.py / fli_bridge.py don't change. Single-cabin path is preserved bit-for-bit; only len(cabins) > 1 takes the new orchestrator.
• PP overlay: when --cabin is multi and --provider-opt pp.cabins= isn't set, PP cabins are auto-derived from --cabin with a Business → +First rule (award seekers treat biz/first as a paired premium tier; First availability is rare enough that surfacing it costs nothing).

Demo

Cash side, gflight backend (flight search JFK LHR --dep 2026-08-15 --return 2026-08-22 --cabin economy,business -n 3 --cash-only):

                Google Flights · Y+J (sorted by Y) (USD)
┏━━━┳━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━┳━━━━━┓
┃ # ┃ carriers ┃ outbound               ┃ return                ┃    Y $ ┃ J $ ┃
┡━━━╇━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━╇━━━━━┩
│ 1 │ ?        │ JFK→LHR VS10           │ LHR→JFK DL2           │ 794.00 │   — │
│ 2 │ ?        │ JFK→LHR VS10           │ LHR→JFK VS137         │ 794.00 │   — │
│ 3 │ ?        │ JFK→LHR VS10           │ LHR→JFK VS25          │ 794.00 │   — │
└───┴──────────┴────────────────────────┴───────────────────────┴────────┴─────┘

— is expected when each cabin's top-N covers different carriers (JFK-LHR: cheapest econ is VS, cheapest biz is FI — distinct top-N sets). Spec acknowledges this; help text suggests bumping -n for broader overlap. Internal N-bump is filed as follow-up work-r9ht.

PP overlay (with --providers pp) auto-shows Economy / Business / First columns — Business → +First rule:

Leg: outbound JFK→LHR 2026-08-15
┏━━━━━━━━┳━━━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━┳━━━━━━━━━━┓
┃ flight ┃     price ┃    Economy ┃   Business ┃ First ┃ ¢/mi (Y) ┃
┡━━━━━━━━╇━━━━━━━━━━━╇━━━━━━━━━━━━╇━━━━━━━━━━━━╇━━━━━━━╇━━━━━━━━━━┩
│ VS10   │ USD794.00 │      36.0k │     100.0k │     — │     1.8¢ │
└────────┴───────────┴────────────┴────────────┴───────┴──────────┘

User can override with --provider-opt pp.cabins=Economy to scope PP independently.

Implementation

• New src/flight_cli/_multi_cabin.py (~110 lines): itinerary_key, MultiCabinRow, merge, parse_price.
• cli.py additions: _resolve_cabin_list, --sort option, _derive_pp_cabins, _run_matrix_multi / _run_gflight_multi (parallel fan-out via anyio.create_task_group), _render_multi_cabin_search, _run_matrix_path_multi / _run_gflight_path_multi top-level entries.
• Per-cabin query failures are soft: log a warning and render that cabin's column as — rather than failing the whole search.
• Matrix deep-link emits for the sort cabin (only cabin Matrix's URL encodes one of).

Test plan

• make check — ruff + ruff format + basedpyright strict + pytest, all green
• 33 new unit tests in tests/test_multi_cabin.py (CSV parsing, key construction, merge with full/partial/no overlap, sort-by-missing sinks, top_n truncation, PP cabin derivation rules)
• Single-cabin regression smoke: render bit-identical to main
• Multi-cabin gflight smoke: --cabin economy,business -n 5 --cash-only
• --sort flip: --sort business reorders rows and updates title
• Multi-cabin matrix smoke: --cabin economy,business --routing "BA"
• Multi-cabin + PP smoke: shows auto-derived Y/J/F PP columns
• PP cabin override smoke: --provider-opt pp.cabins=Economy scopes PP without affecting cash
• Bad --sort cabin: clean exit 2 with message

Deferred (follow-up bd items)

• work-r9ht — internal N-bump for broader join overlap (spec's "Future option").
• work-338g — unified cash+PP super-table render (PP stays per-leg today; rolling per-leg awards up to per-itinerary columns is its own design problem).