fix(formats): dropped is not degraded, and one word for both was a lie (#158)

[JArmandoAnaya]

Closes #158. First of the 0.0.1-beta.2 defect fixes, and the one that silently corrupts a training set.

The defect

check_export reported polygon classes as excluded from a YOLO or VOC export. Both exporters then wrote those polygons anyway, as their axis-aligned bounding boxes. A user consented to losing two annotations and received two extra boxes in their training labels under the polygon's own class name — every row well-formed, every class index real, every coordinate in range, and not what the report said.

Exporter.supported_geometries carried one meaning and was read with two intents, both defensible:

• _compatibility read "not in supported_geometries" as will not be present in the output.
• yolo/_as_box and voc/_as_box read it as convert to something I can write — the capability formats: YOLO exporter — rewritten with v1's math as reference, classes from the schema (not snapshots), read errors never swallowed #62 and formats: VOC exporter (if budget allows; otherwise post-beta) #414 deliberately included, documented in both module docstrings.

So this was never a typo. The model was missing a state: there was no vocabulary for carried, but degraded.

The decision

Option B, argued on the issue: give the port a third state rather than delete the conversion. Option A would have made the report true by removing polygon-derived boxes from YOLO and VOC, which is a real detection-training use.

What changed

Exporter	new degraded_geometries — {polygon} for yolo/voc, empty for coco (writes a polygon as a polygon) and dummy (writes nothing). Disjoint from supported_geometries; supported wins if a plugin says both.
ClassCompatibility	supported: bool → status: supported / degraded / dropped. supported and carried survive as properties, so no report carries a pair that disagree.
ExportCompatibility	excluded_annotations/excluded_assets count dropped only; new degraded_annotations/degraded_assets; new degraded property beside excluded.
reason	says what actually happens: "yolo writes a polygon as its bounding box; the shape is lost" versus "yolo cannot place a classification_tag and drops it".
compatible	unchanged in meaning — false for degraded or dropped, so the allow_lossy gate did not move. Only the accounting became true.
GET /formats	publishes degraded_geometries, because geometries: ["bbox"] alone reads as the whole answer and for yolo it left out that a polygon is written at all.
visionset export	two lines instead of one — what disappears and what arrives coarser are different decisions.

The kernel does not know how a plugin reduces a geometry, so the specific sentence is written only when the format's own declarations make it inevitable (it reduces a shape and can write a box). Anything else gets the general sentence rather than a guess.

The guard, which is the point

tests/formats/test_report_agreement.py (new) exports one release holding 3 boxes, 2 polygons and 1 tag through every installed exporter and counts what each actually wrote — label rows resolved through data.yaml's own index, <object> elements by <name>, COCO instances by category_id. It asserts:

• excluded_annotations equals everything held minus everything on disk — arithmetic over the artifacts, never a restated expectation;
• a dropped class contributes nothing, a degraded or supported class contributes all of its annotations;
• a written class's reason does not say "cannot".

Verified by mutation: restoring yolo's old declaration (degraded_geometries = frozenset()) turns three named tests red, including the two acceptance-criterion ones.

COUNTERS plus a declared NON_WRITING set must cover every registered format, so a fourth exporter either gets compared against its own output or is deliberately exempted — criterion 5.

Ledger

• No migration. FORMAT_VERSION stays 12; VERSION stays 0.0.1b1.
• openapi.json and frontend/ui-core/src/generated/api.ts regenerated together, never hand-edited. This is the milestone's one permitted spec move.
• Kernel purity and the delivery-sibling contract both still KEPT.
• Breaking for API clients: ClassCompatibilityOut.supported → status, and ExportCompatibilityOut gains two fields. Noted in CHANGELOG.md.

Checks run locally

• uv run pytest tests/ — exit 0, 2059 collected
• uv run mypy src/visionset — no issues, 116 files
• uv run ruff check / ruff format — clean
• uv run lint-imports — 2 contracts kept, 0 broken
• pnpm -r build, pnpm test (711 annotator + 107 ui-core vitest, 22 node:test), pnpm -r lint, pnpm -r typecheck — all green

Docs

docs/releases.md gains "Dropped is not degraded, and one word for both was a lie" with the argument and the guard, so it is not re-litigated; docs/tutorial.md shows both CLI lines; CHANGELOG.md gains an Unreleased entry.