Fix #522: SVG round trips keep hidden objects and data-* attributes

[bflatastic]

An SVG animation rig now survives a round trip: hidden alternates stay in the file as display="none", and their data-* attributes (pivots) are kept.

• Hidden objects: SVG's hiddenLayers option now keeps hidden objects as well as hidden layers, each written hidden on its own element (display="none" on the path, image, text or group, in every styling mode). The PSD exporter's option of the same name already covered both. document.save / Save As sets it unless told otherwise, as before for layers, so an SVG Save As keeps hidden objects by default. Plain exports (Export As, document.export) still leave them out unless asked. The CLI's convert asks, since a conversion keeps the document.
• data-* attributes: they are kept as the object's own data, in a new ObjectAttributes.data field (an ordered list of key/value pairs). It is empty in older files and saved by the native format. Our own data-name and data-vc-* attributes are left aside.
  • Import keeps them on the objects and layers their elements become, at most 64 per element.
  • Export writes them back.
  • object.setProps {data: {key: "value" | null}} sets or removes them. A data- prefix on the key is optional, and keys are checked.
• Tests:
  • svg hidden_objects_and_data_attributes_round_trip uses the issue's rig plus a lone hidden path. A plain export leaves the hidden objects out. With hiddenLayers, in all three styling modes, they are written hidden, are not drawn by resvg, and reopen hidden with all their data-*.
  • engine object_data_is_set_saved_and_written_to_svg covers set, remove, undo in one step, rejected keys, SVG output and the native save/load.
• Docs: docs/mcp.md (SVG options paragraph), the CLI convert help text, ROADMAP (SVG bullet and Last updated).

With the issue's rig-repro.svg, vectorcraft-cli convert rig-repro.svg out.svg now writes character, arm, arm-raised and eye-closed, the last two with display="none", and data-pivot on character, arm and arm-raised. Converting the output again only changes the title. Saving as .vectorcraft and converting back keeps everything.

Checks

• Rebased on origin/main 207bd09.
• cargo test -p vectorcraft-svg -p vectorcraft-doc -p vectorcraft-engine -p vectorcraft-format -p vectorcraft-cli -p vectorcraft-mcp -p vectorcraft-ui-egui, cargo clippy on svg/doc/engine/cli with --all-targets -- -D warnings, cargo fmt --all, cargo xtask wasm.

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