feat(design): inspector corner radius for rectangles and frames

[prakashUXtech]

Closes #532

This is slice 1 of #532: corner radius. Fill opacity, stroke and Auto Layout moved to #752, so closing #532 doesn't drop them.

What changed

• DesignModel/DesignNode.swift: DesignNode.cornerRadius (default 0). A hand-written init(from:) decodes it with decodeIfPresent ?? 0, so files saved before this change still open, with square corners. New error DesignTreeError.noCorners.
• DesignModel/DesignCorners.swift (new):
  • DesignNodeKind.hasCorners is true for rectangle and frame only. cornerlessHelp holds the reason line for every other kind.
  • DesignNode.maxCornerRadius is half the shorter side. drawnCornerRadius is the stored radius kept within that maximum, so a layer resized smaller still draws a valid shape.
  • DesignWorkspace.setCornerRadius(_:_:) clamps to 0...min(w, h)/2 and ignores non-finite values. It goes through editNode, so a locked layer, or one inside a locked group, refuses. It coalesces on radius-<id>, so a scrub run is one undo step, the same as the frame fields.
  • cornerRadiusHelp returns the reason the row is off: nothing selected, no corners, or the lock line from lockHelp.
• DesignModel/DesignFile.swift: a file with a negative or non-finite radius is refused as invalidNumber.
• DesignApp:
  • The inspector's Frame section has a new Radius row, a SuiteUI ScrubField labelled "Corner radius". When cornerRadiusHelp is non-nil the row is .disabled and shows that reason as .help.
  • New intent .setCornerRadius, wired in DesignWindow.
  • The canvas fills (and for frames, outlines) rectangles and frames with Path(roundedRect:cornerRadius:style: .circular) at drawnCornerRadius × zoom.
• C4: added DesignCorners and the radius to the DesignModel description.

Tests

Tests/DesignModelTests/DesignCornerRadiusTests.swift runs on Linux. I wrote it before the code, so it failed first. It covers:

• the clamp to half the shorter side, plus negative, NaN and infinity
• the drawn radius after a shrink
• a 20-edit run is one undo step, and undo/redo restore it
• frames take a radius
• ellipse and group refuse with noCorners, and an unknown id is refused
• a layer inside a locked group refuses and nothing changes
• each cornerRadiusHelp reason (nothing selected, ellipse, group, text, group lock, own lock)
• an old file with no cornerRadius opens with 0
• save → open round trip
• a negative radius in a file is refused

scripts/ci prints CI: PASS on Linux after rebasing on origin/dev.

Journey: UITests/DesignJourneyTests.swift has a new testRoundARectanglesCorners. It draws a rectangle, types 24 into Radius, expects "Undo Change Corner Radius", then undoes.

Snapshots (new, from DesignInspectorSnapshots.roundedCorners): design-inspector-radius-light.png, design-inspector-radius-dark.png, design-canvas-radius-light.png, design-canvas-radius-dark.png.

Known limit: ScrubField's onEnd is not public, so the view can't end a run when a drag ends. Two scrubs in a row on the same layer's radius therefore merge into one undo step. setNodeFrame behaves the same way today. Any other edit or an undo ends the run.

Docs: I checked Path on Apple's developer docs (apple-docs MCP) for init(roundedRect:cornerRadius:style:). No other new platform API is used.

Belt-Run: act-1a137c2086a-voa8

Fast-Check: pass
Head: 9fd3950

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Generated by Claude Code

[github-actions]

Fast lane: not merged, this PR conflicts with dev at head 9fd3950. Rebase on origin/dev, rerun scripts/ci, and update the Fast-Check marker.