feat!: a trip's parts carry their own dates and place

[aleskiontherun]

SPEC_REF pins the specification's merged trip-parts commit and schema/, fixtures/ and docs/ are vendored from it byte-for-byte, so a trip is a sequence of §6.9a parts — each with its own optional starts_on, ends_on and nested location — rather than one span beside a flat list of places. The UDDF reader returns a part per <trippart> instead of collapsing the file's dates into min/max, no longer drops a trip for want of a starts_on, keeps the dates of a <trippart> whose <geography> has no <name>, and reads a <trippart> carrying neither a name nor a date as no part at all — which is what lets the writer's mandatory floor element round-trip a partless trip exactly. Where a <trippart> is both of those at once the whole part goes, and the finding says so rather than reporting that the part survived. The writer emits one <trippart> per part with its own <dateoftrip>, omitting that element where a part has neither date and writing a single date into both of its required attributes where it has one. §3's ends_on ≥ starts_on and the bounding-box check move onto trips[].parts[], reporting at trips/<i>/parts/<j> and trips/<i>/parts/<j>/location/bbox.

Verified with pytest on Python 3.10 and 3.14, divejson conform fixtures --strict, the wheel-from-sdist build with its installed-schema check, and a local replay of CI's byte-equality and ancestor checks against the pinned specification commit; .github/scripts/release_bump.py derives 0.9.0 from this branch.

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