M8.19: blend spine and keys editable on the canvas

[bflatastic]

A blend's spine and its key objects can now be edited directly on the canvas.

M8.19: spine and keys on the canvas

• Spine editing
  • A selected blend shows its spine.
  • Direct Selection drags spine points and handles.
  • The Pen tool adds and deletes points on the spine of a selected blend.
  • New commands: object.blend.spine.moveAnchor, addAnchor and removeAnchor. object.blend.info now lists the spine.
• Straight spines become paths
  • On the first edit, a blend's straight spine becomes a real path, with its points pinned to the keys (new append-only field BlendSpec.keyAnchors).
  • Moving a key moves its spine end, and moving a spine end moves its key.
  • Older spines, and spines set with Replace Spine, still spread the keys evenly.
• Picking keys: hit-testing now goes into blend keys (doc/hit.rs). Direct and Group Selection pick keys, while the steps still select the blend as a whole. Edits to a key redraw the blend live.
• Release leaves the spine as an unpainted path below the keys, and the result gains spines.
• Edited spines are kept by Reverse Spine, Reverse Front to Back and adding keys.
• Shared code: the handle logic was pulled into Anchor::set_handle in geom, which path.setHandle now reuses.
• ROADMAP: the M8 row lists this change.

Checks

• This is the last of a six-PR stack on main (M8.12, M8.18, then this PR). Conflicts in shared files were merged token by token, keeping both sides.
• cargo xtask ci passes all 7 steps (fmt, clippy, tests, assets, brands, layers, wasm) at the top of the stack, with 3,178 tests.
• Nothing dropped. object.blend.release's result gains spines; the ids are unchanged.

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