feat(viz): graph_layouts (radial/hierarchical/fixed) + viewport multi-select+lasso (issue 0049i)

Phase 1 — graph_layouts:
- New module cpp/functions/viz/graph_layouts.{h,cpp,md} v1.0.0
- layout_grid, layout_circular, layout_random (migrated from graph_force_layout.cpp)
- layout_radial: BFS rings from root, hop k -> circle of radius k*ring_spacing
- layout_hierarchical: Sugiyama-style heuristic (longest-path levels + barycenter ordering)
- layout_fixed: no-op
- All respect NF_PINNED. graph_layout_circular/grid kept as deprecated wrappers.

Phase 2-3 — graph_viewport v1.2.0:
- Multi-selection via state.selection (vector<int>); NF_SELECTED kept in sync
- Lasso: Shift+Drag on empty area; AABB hit-test on release
- Drag of N-selection: all selected pinned + moved by mouse delta
- Ctrl+click toggle, Esc clears selection
- Right-click on node -> on_context_menu callback
- Double-click on node -> on_double_click callback
- Helpers exposed: graph_viewport_clear/add_to/toggle/is_selected (own TU for tests)

Phase 4 — tests:
- test_graph_layouts: 12 cases / 364 assertions covering geometry, pin, edges
- test_graph_viewport: 5 cases for selection helpers (pure logic, no GL)

Phase 5 — demo (primitives_gallery):
- Layout combo (force/grid/circular/radial/hierarchical/fixed) + Apply button
- Right-click popup with Pin/Unpin/Add-to-selection
- Status overlay shows [N selected] when selection non-empty
- Updated golden images

Issue moved to dev/issues/completed/.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cpp/apps/primitives_gallery/CMakeLists.txt

@@ -70,7 +70,9 @@ add_imgui_app(primitives_gallery
     ${CMAKE_SOURCE_DIR}/functions/viz/graph_icons.cpp
     ${CMAKE_SOURCE_DIR}/functions/viz/graph_force_layout.cpp
     ${CMAKE_SOURCE_DIR}/functions/viz/graph_force_layout_gpu.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/graph_layouts.cpp
     ${CMAKE_SOURCE_DIR}/functions/viz/graph_viewport.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/graph_viewport_selection.cpp
     ${CMAKE_SOURCE_DIR}/functions/core/graph_spatial_hash.cpp
     # GL loader (Linux no-op, Windows wglGetProcAddress)
     ${CMAKE_SOURCE_DIR}/functions/gfx/gl_loader.cpp