PartOps Slice F — Docs, tests, and workflow polish

[fernandotonon]

Goal

Make the full PartOps workflow discoverable, documented, and regression-safe.

Parent epic: #859

Scope

• Add README/CLAUDE docs for AI segmented parts.
• Add UI copy/tooltips for preview, split, explode, join, and print prep.
• Add Sentry breadcrumbs:
  • mesh.parts.segment_preview
  • mesh.parts.split_segments
  • mesh.parts.explode
  • mesh.parts.join
  • mesh.parts.print_connectors
• Add screenshots/GIFs to the epic or docs.

Acceptance Criteria

• Edit Mode clearly exposes the workflow.
• Docs explain limitations: body-centric labels, simple alignment pegs, model-unit dimensions.
• All PartOps commands have headless-safe tests.
• Existing Select by Part, SceneTree submesh selection, and topology editing tests still pass.