M8.17: Edit envelopes on the canvas (M8 done)

[bflatastic]

Envelopes can now be edited on the canvas: Edit Contents, mesh points and handles, and Control bar controls. With this, all 20 M8 tasks have landed, and the ROADMAP marks M8 done.

M8.17: editing envelopes on the canvas

• Edit Contents. While you edit an envelope's contents, clicks hit the content where it sits undistorted (new Hit.contents_of), and the menu label toggles to Edit Envelope.
  • It has no default shortcut: the reference app's Shift+Cmd+V is Paste in Place here. This is documented and tested.
• Mesh on the canvas. A selected envelope shows its mesh (the warp surface grid, the mesh grid, or the top object's outline) instead of the content's outlines.
• Mesh handles. Mesh envelopes get bezier handles in a new append-only field, EnvelopeKind::Mesh.handles.
  • When empty, the envelope keeps today's Catmull-Rom map. The first handle edit fills the handles in exactly along the grid lines.
  • Round-trip and old-file tests cover this.
  • setMeshPoint takes handle.
  • object.mesh.movePoint, addLine and deletePoint also edit mesh envelopes.
• Tools. A new shared tools::meshedit::MeshEdit lets the Mesh tool and Direct Selection drag points and the clicked point's handles. The Mesh tool also adds rows and columns inside mesh envelopes.
• Reset and Release. Reset with Mesh keeps the shape, including its handles, and Release returns the exact mesh.
• Control bar. New envelope controls: Edit Envelope / Edit Contents, warp style, orientation, bend and H/V distortion (or mesh rows and columns), Reset, and Envelope Options. A headless frame test clicks them.

ROADMAP and README (M8 is complete)

• M8 row: ✅ done, listing what the fidelity pass delivered. What's left: Anti-Alias and Preserve Shape output, editing warp-envelope points, brush interpolation in blends, and Free Transform/Scale/Reflect along a rotated box.
• Parity row (Symbols, blends, envelopes, Repeat, perspective): 75% → 88%; remaining work 8–12 h → 5–8 h.
• Feature parity: ~74% → ~75% (honest range 69–75%), ~225–345 h. Full parity ~56%, ~330–515 h.
• "Shipped so far" and the README status line are updated to match.

Checks

• Rebased on main after M8.11: Perspective grid widgets: ground-level points move the grid, separate extents, cell size widget, screen-fixed Plane Switching Widget with 1-4 keys #201. The one conflict, in docs/control-protocol.md, was merged keeping both sides.
• cargo xtask ci: all 7 steps pass (fmt, clippy, tests, assets, brands, layers, wasm), 3,276 tests.
• Nothing dropped: compared with main, the branch keeps every command, UI command, menu item, MCP tool and test, and adds 9 tests.
• Changed tests:
  • the EnvelopeKind::Mesh literals in tests_envelope gain handles: vec![];
  • object.mesh.movePoint now rejects a handle that isn't an integer or is out of range, instead of ignoring it.
• docs/mcp.md code fences are balanced.

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