Write generic keys instead of dive_-prefixed ones

[mattdawkins]

Per-detection attributes, track attributes, notes and confidence pairs are generic annotation concepts — nothing about them is specific to DIVE — so export should not brand them. Export now writes attributes, track_attributes, notes and confidence_pairs.

Import already fell back to the unprefixed names for attributes and notes (kwcoco.py:345,349, coco.ts:452,462,466); this makes those the preferred spelling and keeps the dive_-prefixed ones as legacy aliases, so files DIVE wrote earlier keep importing. confidence_pairs gains the same treatment on both readers.

Kept prefixed, because they really are DIVE-specific:

• info.dive_dataset_info — DIVE's dataset metadata block
• info.dive_extensions — DIVE describing which optional keys it used (its contents are now the generic names)

This changes DIVE's output format. Anything downstream reading dive_detection_attributes, dive_track_attributes, dive_notes or dive_confidence_pairs off a DIVE export needs updating; the readers here are covered, third parties are not. info.dive_extensions still advertises which keys a given file uses.

Tests: export assertions moved to the generic keys; existing prefixed documents were left as-is so they now serve as legacy-import coverage, and both suites gained an explicit generic-vs-prefixed pair. docs/DataFormats.md updated, including the worked example and the legacy-alias notes.

Verified: 44 server tests pass locally, and a real export/import round trip writes ['attributes', 'confidence_pairs', 'notes', 'prob', 'track_attributes'] and reads every value back.

Not run: vitest, eslint and tsc — no node_modules in this checkout.

Stacked context: #1865 renames videos[].fps to annotation_fps and is still open. Both touch these files, so whichever lands second will need a trivial rebase.