Add segmentation annotation and import

[alexschmander]

Problem

Labello supports bounding boxes and skeletons only; segmentation geometry cannot be authored, reviewed, imported, replayed, or exported.

Current behavior

No end-to-end segmentation domain geometry, canvas interaction, event/wire contract, persistence, assignment, review, statistics, or supported import profile exists.

Desired behavior

Implement a versioned segmentation capability with usable annotation/review tools and explicit supported import semantics.

Acceptance criteria

• Choose and document supported polygon/mask representation, bounds, topology, holes, precision, and resource ceilings.
• Add pure validation, versioned wire/events, replay, persistence migration, snapshots/offline compatibility, and provenance.
• Provide accessible canvas creation/editing, undo/redo, selection, review/correction, and responsive behavior.
• Add explicit supported import profiles with plan/build/verify/no-replace recovery.
• Cover malformed/complex geometry, large masks/polygons, rendering/hit testing, API limits, statistics, and interoperability.

Scope

New annotation type end to end; exports/round trips only where explicitly included by their separate contract.

Implementation notes

Avoid inferring support from adding an enum variant alone; resource bounds are a core trust-boundary requirement.

Tracking source

docs/tracking/feature-requests.md:44

Segmentation annotation and import.