feat(lean): randomize phase folding

[barghouthi]

Summary

• model phase folding as a RandPass over uniform finite samples
• prove the failure event is bounded by the affine-tag collision probability
• draw fresh 128-bit tags from the OS in the executable pipeline
• compose randomized passes and fixpoint rounds with conditional fresh seeds
• remove the exact phase-fold fallback so PhaseFoldRand is the default implementation

Verification

• lake build (5269 jobs)
• PhaseFoldRand CLI smoke test
• git diff --check