fix: prevent volatility surface label clipping

[fallblu]

Summary

• keep every sampled volatility-surface expiration label inside 1200 x 720 PNG exports
• default all built-in Plotly figures to 900 pixels tall while retaining caller overrides
• document and regression-test the layout policy

Refs #304

Test plan

• make lint type test docs-check
• make docs-build
• make package-check
• uv lock --check
• render and inspect a 15-expiration surface with write_image(..., width=1200, height=720, scale=1)