Allow curve tree nodes to be both channels and parents

[phfix]

Summary

Fix a curve tree collision when a channel path is also a parent of other channels.

Details

When / separators are used, names like input/a and input/a/ad_raw can collide in the tree representation.

The issue is order-dependent: if input/a/ad_raw is inserted first, the input/a node is created as a parent branch, and the later insertion of the real input/a channel is effectively hidden.

This change allows a tree node to represent both a real channel and a parent node, so the shorter path is no longer lost.

A small CSV sample was added to datasamples/ to reproduce the issue.

Notes

This keeps the current drag behavior unchanged. If subtree dragging is added later, branch-vs-leaf behavior will likely need to be made more explicit in the UI.

Filtering on a node that is both a channel and a parent can still be somewhat ambiguous, but the current behavior is consistent with preserving both roles on the same row.

Possibly related to other path/tree representation issues such as #622, although the underlying cause here appears to be different.

[facontidavide]

funny enough, I detected that yesterday. let me have a look at your proposed change

[facontidavide]