Fix #798: the Curvature tool edits any selected path

[bflatastic]

Fixes #798.

The Curvature tool only edited the path it was drawing. It now works on any selected path, whatever drew it, and changes nothing outside the edit:

• Select and drag a point: a click makes it the current point (direct-selected, filled, with its handles). A drag moves it, with snapping, and only its two segments change: a smooth point turns along the line through its neighbours, the neighbours keep their handle directions (lengths scaled to the chord), a corner keeps its handles. Segments further away are bit-identical.
• Toggle and delete: Alt-click or double-click toggles smooth/corner (path.convertAnchor); Delete/Backspace removes the current point (path.removeAnchor). The tool now claims Delete while it has a point to remove: before, Delete removed the whole path even while drawing.
• Add a point: a click on a segment adds a smooth point with an exact split, so the shape doesn't change; dragging at once moves it in the same undo step.
• Continue and close: while the current point is an end of an open path, clicks extend it (the old segments keep their shape; Alt places a corner); a click on the other end closes it. Esc/Enter drops the current point. New paths are drawn as before.
• Cursors for close, continue, add, convert and move; a rubber band while continuing.

New command: path.curvatureEdit {id, subpath?, op: "move"|"insert"|"extend"|"close", anchor?, segment?, t?, end?, from?, x?, y?}, one undo step, every parameter checked, so agents can make the same edits (docs/mcp.md › Editing a path with the Curvature tool). editable_paths moved from the Pen to draw2 so both tools share it.

Tested:

• cargo xtask ci (all 7 steps), on its own and stacked with PNG export: anti-aliased edges of semi-transparent art come out at alpha 254 over an opaque background, so the file is written as RGBA #787.
• 11 tool and geometry tests in curvature.rs (moved points leave other segments bit-identical; continuing and closing keep old segments; inserting keeps the shape), an engine test that each edit on a Pen path is one undo step, and junk-parameter cases in command_sweep.rs.
• Live: a 5-point Pen path, dragged anchor 2 with the Curvature tool: anchors 0, 3 and 4 unchanged, anchor 1's out handle in the same direction; one "Curvature" history step. Continuing from the end, adding a point on a segment, Delete and double-click each made one step.

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