feat(kit): SeatMap — named-seat reservation for a fixed venue layout (FT315)

[hideyukiMORI]

Summary

Field Trial FT315 — Nene\Kit\SeatMap: assigned seating over a fixed venue layout. Distinct from EventTicket (capacity count), ResourceReservation (time-bounded), TimeSlot (appointments).

Method	Description
addSeat / addRow	Define seats (idempotent).
reserve(venue, seat, holder): bool	Claim a free seat; false if taken.
release(venue, seat): bool	Free it.
holderOf / isAvailable / availableSeats / reservedSeats / seatsOf	Reads.

• Race-free claim: single guarded UPDATE … WHERE … AND holder IS NULL.
• Idempotent seat definition never wipes a holder; reserving a nonexistent seat throws.

F-N findings

• F-1 — seat names sort lexically ("A10" before "A2"); zero-pad for >9-seat rows. Documented; helper stays storage-order-agnostic.

Test plan

• 12 tests / 25 assertions (reserve/holder, double-reserve guard, release+re-reserve, available/reserved listings, seatsOf, idempotent addSeat keeps holder, venue separation, nonexistent-seat throw, safe unknown reads, validation)
• composer precommit green (format → Phan → 5192 tests)

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