Cost right-sizing: g4dn.xlarge + spot for lecture-python.myst PR previews

[mmcky]

#92 Phase 3: cost right-sizing for lecture-python.myst PR previews — g4dn.xlarge + spot for previews only (weekly/publish builds unchanged).

• A/B a full cold run in 16 GB first
• Scope spot to PR previews — QuantEcon/workspace-lectures#6 wants on-demand (spot=false) for long-running and GPU builds, so the boundary needs stating

Arithmetic in PROJECT-OPTIMIZE-PREVIEWS.md.