M4: SVG import keeps symbols, patterns and filters live; linked images, nested clips, reflected gradients (M4.41, M4.42)

[bflatastic]

Part of M4 (Files). SVG import keeps symbols, patterns and blur/shadow filters live, finds linked images in the SVG's folder, intersects nested clips, keeps hidden objects and expands reflected gradients.

• M4.41: symbols, patterns and filters stay live.
  • Symbols: <symbol> with <use> imports as one symbol (named from data-name, else its id) with an instance per <use>.
    • A <use> stays plain art when it would look different as an instance: paint inherited from the <use>, a viewport that cuts the art, or a transform that would change its look. This uses the export's rules from M4: SVG symbols as <symbol>/<use>, hidden layers, metadata; Preserve Editing round trip (M4.39, M4.40) #72, now shared.
  • Patterns: <pattern> imports as a pattern swatch (already on main; now covered by a checker test).
  • Filters: feGaussianBlur, feDropShadow and the common shadow chains (offset / blur / flood or colour matrix / composite / merge) become live Gaussian Blur, Drop Shadow, Outer Glow, Inner Glow and Feather effects. Other filters still warn.
  • Round trip: export writes data-name on <symbol> and data-vc-blend on shadow and glow filters, so export → import → export is stable.
• M4.42: linked images, clips, hidden objects, reflected gradients.
  • Linked images: document.open {path} and Place find an SVG's relative <image> links in its folder (or the folder in name with dataBase64).
    • Raster files stay linked, with a full LinkInfo and a preview (M4: linked images with previews, missing and modified links; Text Import Options (M4.53, M4.54) #78).
    • SVG files and data: SVGs import as vector art.
    • An unreadable file becomes a placeholder that keeps the link, with a warning and a missingLinks row; links.update and links.relink fill it in later.
  • Library API: ImportOptions {folder, read} and import_with. The library itself no longer reads any file; before, hrefs were resolved against the working directory.
  • Other import changes:
    • Nested clip paths intersect.
    • Every display:none object comes back hidden.
    • spreadMethod reflect and repeat are expanded into stops, matching resvg in a pixel test.

Known limits:

• Non-local hrefs (http and the like) become missing-link placeholders.
• Text inside an SVG used as an image isn't imported (with a warning).
• A blur with different x/y spreads is read as their average.
• A <use> with a filter stays plain art.

Checks

Rebased onto 3e8c8d6 (#93). cargo xtask ci passes all 7 steps (fmt, clippy, test, assets, brands, layers, wasm): 2,393 tests, 0 failed.

• Nothing dropped:
  • every test name on main still exists (2,362 → 2,388);
  • command ids, UI_COMMANDS, menu entries and MCP tools are unchanged.
• Changed tests, from the intended behaviour:
  • hidden_text_is_skipped_and_use_instances_are_kept: hidden text now comes back hidden.
  • three_instances_give_one_symbol_and_three_uses: the read-back is now one symbol with three instances.
  • an_svg_edited_elsewhere_opens_as_plain_svg_with_a_warning: a plain import now has symbols.

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