M4: SVG symbols as <symbol>/<use>, hidden layers, metadata; Preserve Editing round trip (M4.39, M4.40)

[bflatastic]

Part of M4 (Files). SVG export writes symbols as <symbol>/<use>, keeps hidden layers on Save and carries Dublin Core metadata. SVGs saved with Preserve Editing reopen exactly, or as plain SVG once another app has edited them.

• M4.39: symbols, hidden layers, metadata.
  • Symbols: each symbol is written once as <symbol overflow="visible">, and each instance as a <use>.
    • Some instances can't share the definition without looking different from the canvas, so they are written as their own art:
      • instances stained by their fill;
      • scaled instances of symbols with strokes;
      • rotated or scaled instances of symbols with effects, brushes or live objects;
      • instances of symbols with pattern paints or unlinked masks.
    • Instances written as art are stained as on the canvas, and keep their patterns and masks on the page.
    • A symbol that contains itself stops at depth 8 instead of recursing forever.
    • The stain helper moves to vectorcraft_brush::{stain, instance_art}, shared with the renderer.
  • Hidden layers: a new SVG option hiddenLayers (listed in document.formats). Save, Save As and Save a Copy turn it on; exports leave hidden layers out. On import, a display:none group with an id comes back as a hidden layer or group.
  • Metadata: written through one Dublin Core list (dublin_core()), currently the format and title. File Info will fill in the other fields.
• M4.40: Preserve Editing round trip.
  • What is stored: the native document goes inside <metadata> as CDATA, with a hash of the SVG markup around it. Re-indenting and line-ending changes don't change the hash.
  • On open: document.open restores the document exactly when the hash matches. Older SVGs without a hash are trusted, as before.
  • Edited elsewhere: an SVG another app edited, or one whose editing data is damaged, opens as plain SVG and says why in warnings (EDITING_STALE, EDITING_DAMAGED).
  • API: new vectorcraft_svg::editing().

docs/mcp.md covers the option and the round trip.

Known limits (these problems predate this PR):

• The canvas doesn't draw opacity masks inside symbol art.
• Live geometry effects inside symbol art aren't written to SVG or PDF.
• Pattern paints on type inside symbols aren't pinned to the page.

Checks

Rebased onto 4cccbc2 (#71). cargo xtask ci passes all 7 steps (fmt, clippy, test, assets, brands, layers, wasm): 2,091 tests, 0 failed.

• Nothing dropped:
  • every test name on main still exists (2,074 → 2,088), and no existing test changed;
  • command ids, UI_COMMANDS, menu entries and MCP tools are unchanged.
• New tests:
  • <symbol>/<use> output and the cases written as art;
  • link mode writes no data: URLs;
  • hidden layers on Save and on import;
  • a live effect, a symbol and a pattern reopen exactly;
  • an externally edited file falls back to plain SVG;
  • the import fuzz test also mutates saved SVGs.

🤖 Generated with Claude Code