Enhance paste_graph method

[axif0]

Enhance paste_graph method to track new variables added during graph paste operation

Closes: #363

[boldar99]

Hi, thanks for the PR. I have started testing this and there seems to be some issues with the implementaiton:

1. When copying from one tab to another, the parameter types are not preserved:

Screen.Recording.2026-02-17.at.15.38.30.mov

2. This might be an IOS related issue (see Cannot copy-paste diagrams to other ZXLive instances on IOS: #437), but I am not able to paste to another window (even a simple diagram). I see you haven't included a video about this in your PR, but this is something explicitly mentioned in the issue.

[axif0]

GraphScene.update_graph was dropping GraphDiff.variable_types during scene updates, causing boolean and parametric variable types to be lost. This change reapplies the stored variable_types to new_g.var_registry after diff.apply_diff, preserving correct variable type information.

[lia-approves]

Thanks for the PR. The copy paste works for only 1 of the 4 settings: both with and without Shift, on both different tabs and different windows. As there is an earlier PR (#433) that correctly implements for all 4 settings we will most likely award this bounty to that PR, but wish you happy hunting with your other bounty submissions.