Function select_prov_mode 

pub fn select_prov_mode(analysis: &GraphAnalysis) -> ProvMode

Select the optimal provenance mode based on graph analysis.

Heuristic (crates/polydat/docs/design/engines.md):

• Pull has zero overhead on all-dirty graphs (cone check ~2ns)
• Pull is the safe default for selective output access
• PushPull when multiple inputs exist (push skip helps within dirty cones when some subgraphs are stable)
• Raw only for tiny single-input graphs