Add global PnR candidate smoke

[rollrat]

Summary

• add a minimal global PnR skeleton with layout candidates, shelf placement, and world assembly
• add input placement constraints so global placement can narrow local placer input search
• add an ignored D flip-flop smoke that assembles two D latch child candidates and saves an NBT fixture

Verification

• cargo test --release generate_inputs_uses_explicit_position_region -- --nocapture
• cargo test --release local_placer_limits_input_search_to_named_constraints -- --nocapture
• cargo test --release local_placer::unit_tests -- --nocapture
• cargo test --release graph_backed_module_generates_world_with_global_pnr_api -- --nocapture
• cargo test --release d_flip_flop_module_generates_world_from_child_layout_candidates -- --ignored --nocapture