D8.12-A BimCraft: duplicate a view, as its own copy or linked

Today a plan is one view per level plus callouts; there is no Duplicate. Add Duplicate on plans, ceiling plans, sections, elevations and callouts with a choice: Own copy (drafting, dimensions, crop, hidden elements and pen overrides copied; editing or deleting them changes only that view — the default) or Linked (one set shared by both views). The working style (D8.12) sets which one is offered first. Older views on issued sheets must never change because a later copy was edited (the requester's point: revisions are not a straight line). Stored in the IFC as now (CadCraft_PlanView / CadCraft_Drafting View key). Part of D8.12.

**Implementation started 2026-11-02** (0.9 top-5 pick #2, hardest first; session `bim-views-09`, log docs/0.9-sessions/bim-views-09.md). Status moves to Partly built or Built only when the code is reachable and tested (rule 49).