ModularShoot

### Added

- **Install API supports a preferred plugin category**: `installPlugin` gained an optional preferred-type parameter (e.g. a plugin panel dragging a plugin onto a category area). A matching hint installs directly into that category; a miss falls back to the original auto-selection. Existing callers are unaffected.

- **Effective-slot query facade**: new `getEffectiveSlots` API returns each category's effective capacity (including installed plugins' `adds_slots`), so UIs can render install areas without re-implementing the aggregation.