apollo_starknet_os_program,starknet_os: add the Cairo0 proof-fact fold tree

[einat-starkware]

Fourth PR of the privacy proof-fact fold stack. Adds the fold tree above the leaf digest in Cairo0: fold_block_proof_facts folds a block's ProofFactsReference array into the block's root fold entry (leaf entries, pairwise folding with a carried trailing entry, single-transaction self-fold), and compute_fold_digest computes the digest the Cairo circuit verifier outputs for that entry. Still not reachable from the OS program — the OS output wiring lands in #15064.

Tested for bit-exact agreement with the Rust mirror across tree shapes exercising the pairing and carry rules (N = 1, 2, 3, 4, 5, 7).

Stack: leaf digest (#15090) ← rust tree fold (#15091) ← cairo leaf digest (#15092) ← this PR ← OS output wiring (#15064) ← registry pin (#15086) ← verifier task (#15088).

🤖 Generated with Claude Code

https://claude.ai/code/session_01XmPJM3Wph4QLmFmhcxVsh4

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Generated by Claude Code

[cursor]

PR Summary

Medium Risk
Introduces protocol-sensitive fold/digest logic that must match the prover; risk is mitigated by bit-exact Rust tests but integration is still pending.

Overview
Adds the Cairo0 mirror of the privacy proof-fact fold tree so the OS can reproduce the proving side’s recursive digests bit-for-bit.

proof_fact_fold.cairo now exposes fold_block_proof_facts (per-transaction ProofFactsReference → block root fold entry), compute_fold_digest (verifier public output over a 16-word entry), and the supporting leaf build / pairwise fold pipeline—including self-fold for one tx and carry of an unpaired trailing entry. Leaf entries still pair the leaf-verifier circuit hash with compute_leaf_output_digest; internal nodes use the multiverifier hash and BLAKE2s over child entries.

proof_fact_fold_test.rs runs those Cairo entry points through the VM and asserts exact agreement with the existing Rust mirror for tree sizes 1–7 (pairing/carry cases) and for compute_fold_digest. This code is not wired into the main OS program yet (per stack note).

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[reviewable-StarkWare]

This change is