M4: PNG, PDF, SVG and text clipboard formats in and out (M4.57)

[bflatastic]

Part of M4 (Files). Copy offers PNG, PDF, SVG and plain text to other apps, and Paste takes SVG, PDF, text and bitmaps from them, as Illustrator does.

• M4.57: clipboard formats in and out.
  • Copy and Cut also offer:
    • PNG, always, at most 4096 px a side;
    • PDF, with the copyAsPdf preference, without the embedded editing data;
    • plain text: the text when every copied object is type, else the SVG markup when copyAsSvg is on;
    • SVG (image/svg+xml), when copyAsSvg is on.
  • Paste: another app's copy is taken as SVG, then PDF, then plain text (as point text), then a bitmap (embedded at the view centre). VectorCraft's own copy still pastes from the lossless internal clipboard. Ctrl/Cmd+V also pastes a bitmap or PDF that arrives without text.
  • Keep Plain Text: with pasteTextFormatting: "plain", text the Type tool copied pastes without its formatting.
  • New queries (cmd/clipboard/flavours.rs):
    • clipboard.exportPng {scale?}, clipboard.exportPdf, clipboard.exportText;
    • clipboard.flavours, which lists what Copy offers;
    • clipboard.importImage {dataBase64, mime?, center?}, clipboard.importText {text, center?} and clipboard.importPdf {dataBase64, page?, password?, center?}.
  • Platform layer: a SystemClipboard service trait (ui-egui/src/sysclip.rs) with desktop backends (apps/vectorcraft/src/clipboard.rs).
    • Windows uses clipboard-win, which writes every format at once plus an opaque bitmap. It was already a transitive dependency through arboard, under the Boost Software License 1.0, which is permissive.
    • macOS and Linux use arboard with image-data.
    • The web build stays SVG-text only.
  • Behaviour change: if another app's copy can't be loaded (broken SVG, whitespace-only text), Paste now reports the error and pastes nothing, instead of pasting the stale internal clipboard.

docs/mcp.md has a "Clipboard formats" section, and docs/control-protocol.md is updated.

Known limits:

• EMF paste waits for EMF support (M4.95).
• macOS and Linux carry one format at a time (text, else PNG) and paste text and bitmaps only. They were type-checked on Windows; I couldn't run them here.
• A clipboard holding only a bitmap doesn't enable Paste when nothing was copied in the app.

Checks

Rebased onto a560f74 (#79). cargo xtask ci passes all 7 steps (fmt, clippy, test, assets, brands, layers, wasm): 2,274 tests, 0 failed.

• Nothing dropped:
  • every test name on main still exists (2,251 → 2,269), none changed;
  • every command id is present (523 → 530);
  • UI_COMMANDS, menu entries and MCP tools are unchanged.
• Checked in the running app on Windows: the clipboard listed every format, and a bitmap and text from another app pasted correctly.

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