feat(graph): allow direct layout transitions between 3D and linear

[diogobaltazar]

Currently getModeMorphSequence (App.jsx:294) forces an intermediate stop at 2D when transitioning between 3D and linear:

• 3d → linear routes through ["2d", "linear"]
• linear → 3d routes through ["2d", "3d"]

Remove this constraint so both directions morph directly (["linear"] / ["3d"]), matching the existing behaviour for every other pair (e.g. 3d ↔ 2d, 2d ↔ linear).

Acceptance criteria

• Switching from 3D to linear transitions directly, without a visible 2D intermediate
• Switching from linear to 3D transitions directly
• All other transition pairs are unaffected
• No visual regressions on the 2D ↔ linear or 2D ↔ 3D paths