[AI] Clean up AI object mask post-processing

[andriiryzhkov]

Two post-processing steps were originally added to improve the AI mask quality. After testing and user feedback, both turned out to make things worse – removing them.

• _morph_open_close clipped real fine details (hair tips, fingers) and filled real concavities (gaps the model correctly identified). Stamped pixels back to hard 0/1, destroying soft alpha.
• _guided_filter_refine smoothed an already near-binary mask. Confirmed via live tuning: every non-zero radius softened the boundary without improving alignment.

The roles they played are already covered without side effects: _keep_seed_component handles disconnected blobs, potrace's turdsize handles tiny specks.

Result

Post-processing is now: decoder → _keep_seed_component → potrace → feather. Model output reaches the path without displacement or smoothing.

[TurboGit]

Works for me, thanks!