…d loose reset for attention mapping, and improved scipy dependency handling

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

…matrix

libinchi is C-only so one build per OS suffices. A dedicated libinchi job
produces an artifact per platform; the binary job downloads it before
building wheels, eliminating CMake from the Python matrix entirely.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

…, size filter, purification bail

- Protection is now tried first (step 1) before size filter, since it can legitimately produce
  large products from unlisted reagents
- Purification bail-out added (cheap equality check before all template enumeration)
- max_size_ratio / min_filter_size parameters added to skip grossly imbalanced records
- deprotect_excess now composes with both single-molecule transforms and multi-component reactions
  by building a candidate-forms table (raw + deprotected) per reactant
- _try_single / _try_deprotect_then_transform removed; unified as forms × _try_transforms + _try_multi
- iproduct used to enumerate all raw/deprotected form combos in the multi-component path
- remap applied directly to reactants[i] (not via _safe_remap on the product side)
- reset_mapping guarantees disjoint reactant/product atom number spaces before reconstruction

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>