docs: a trip is a sequence of parts, each with its own place and dates

[aleskiontherun]

feat(trips)!: each part of a trip has its own place and dates (#274) removed a trip's own start_date/end_date and its flat locations[], but left prose behind that still describes the old shape. The README's Trips bullet promised "location and dates"; Course's API doc and the course dialog's end-date comment both drew a contrast against "a trip's start date", which no longer exists and whose part-level replacement is optional on both ends exactly as a course's dates are; formatDateOnly and formatTripDateRange in lib/date-time.ts named a trip as the owner of dates a part now owns; and the re-ordering notes in useInfiniteResource.ts and its test said trips sort by start date, where get_trips_page orders them by the earliest start across their parts — an aggregate over trip_part, not a column of trip. The paragraph's point survives: that date is still one the edit dialog can change, so the row still moves.

Swept the whole repository, not just src/, reading every trip hit with each file squeezed to one line so a formatter's wrap could not hide a claim. DECISIONS.md was already rewritten by #274 and its cross-references still resolve; CONTRIBUTING.md, scripts/measure-navigations.mjs, privacy and terms carry only "round trip" or claims that stayed true. Nothing about dive sites is touched.

Verified with npm run format:check, npm run lint, npm run type-check and npm test (220 files, 7144 tests, all passing).

🤖 Generated with Claude Code