starknet_os: add the privacy proof-fact fold tree

[einat-starkware]

Second PR of the privacy proof-fact fold stack. Builds the fold tree above the leaf digest, in Rust:

• FoldEntry (circuit hash + output digest, 16 u32 words) and the two canonical_small circuit-hash constants (leaf verifier, multiverifier; placeholders until the production registry lands).
• fold_block_proof_facts: leaf entries folded pairwise left-to-right with a carried trailing entry; a single transaction self-folds, so the block root is always a multiverifier node.
• compute_fold_digest: the digest the Cairo circuit verifier outputs for a proof whose facts fold to a given entry.

Tested against the proving side's four_leaves golden fixture (a real proven tree).

Stack: leaf digest (#15090) ← this PR ← cairo leaf digest ← cairo tree fold ← 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

Low Risk
Self-contained digest/fold logic with golden tests; circuit hashes are explicit test-registry placeholders and callers are not wired in this PR.

Overview
Extends proof_fact_fold with the privacy proof-fact fold tree above the existing leaf digest: a FoldEntry (leaf/multiverifier circuit hash + Blake2s output digest), placeholder canonical_small registry hashes for leaf verifier and multiverifier, and fold_block_proof_facts to turn each transaction’s proof facts into leaf entries and fold them pairwise (trailing odd entry carried; a single tx self-folds so the block root is always a multiverifier node). Also adds compute_fold_digest (verifier output over the entry’s 16 u32 words) and internal fold helpers that hash pairs of children’s raw 32-word preimages.

Adds a four_leaves golden test aligned with the proving side’s recursive-tree fixture, alongside the existing leaf-digest golden.

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[reviewable-StarkWare]

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