feat(viz): Plotly cross-section + BPP top view + gasket outline (Phase 3)

Phase 3 of Visual Stack Designer (v0.4). Also fixes ruff format nit in
test_components.py that broke Phase 2 CI (ruff format --check was stricter
than ruff check).

src/visualization.py (NEW):
- draw_layer_cross_section(assembly, max_visible_cells=6): side-view stacked
  bar chart in mm. For N > max_visible_cells, shows 3 cells top + "... X
  collapsed ..." box + 3 cells bottom to keep Plotly shape count bounded at
  high N (N=200 smoke-tested).
- draw_bpp_top_view(bpp, active_area, gasket): SVG-shape flow-field renderer.
  3 patterns (serpentine meander, parallel fingers, interdigitated alt.
  inlet/outlet). Ports as diagonal-corner circles.
- draw_gasket_outline(gasket, active_area): standalone gasket with cut-out.
- draw_assembly_summary(a): 2-panel placeholder for export (v0.4 minimal).
- No new runtime deps: plotly is already in requirements.

tests/test_visualization.py (NEW, 5): figure creation for N=1 and N=200
(compressed-view label check), all 4 BPP presets render shapes, KeyError
on unknown BPP or gasket, gasket outline has 2 shapes (outer + cutout).

122 tests green (was 117, +5). Ruff check + format both clean.

Author: Tools00