apollo_starknet_os_program,starknet_os: add the Cairo0 processed-proof digests

[einat-starkware]

Part 3 of the single-proof verification stack (single transaction, single proof facts; no combining).

The Cairo0 mirror of #15091: the proof output digest over a transaction's proof facts, the processed proof's output digest over two verified proofs' facts (matching the Rust signature — the single transaction's caller passes the same proof facts into both of the multiverifier's verifier slots), the verification digest, and the two circuit hash constants. Straight-line computations — no entry structs, no circuit tables or indexes. Compiled standalone as a test program; not yet reachable from the OS program (that lands in #15064).

The Cairo leaf, processed-proof and verification digests are tested against the proving side's goldens, and the leaf combination also against the Rust mirror over two different leaves, which the golden can't distinguish.

Stack: #15090 ← #15091 ← this PR ← #15086 ← #15164 ← #15161 ← #15166 ← #15167 ← #15168 ← #15169 ← #15163 ← #15064 ← #15162

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https://claude.ai/code/session_01XmPJM3Wph4QLmFmhcxVsh4

[cursor]

PR Summary

Low Risk
Test-only Cairo mirror and VM parity tests; no changes to production OS execution paths or auth/data handling.

Overview
Adds a standalone Cairo0 proof_fact_fold test program that mirrors the existing Rust proof-fact folding logic for single-proof verification: leaf output digest from proof facts (Blake2s over program-hash onward), processed proof output from two leaf digests via the leaf verifier circuit hash, verification digest from the multiverifier circuit hash, plus pack_output_digest and hard-coded leaf/multiverifier circuit hashes.

The build wires proof_fact_fold.cairo into compile_test_contracts and exposes PROOF_FACT_FOLD_BYTES for tests. proof_fact_fold_test now runs each digest function through the Cairo VM and asserts bit-exact agreement with Rust on proving-side goldens, and adds a combine_leaf_digests case with two different leaves so swapped-slot bugs are not masked by the symmetric golden.

The program is compiled for tests only; it is not linked into the main OS program yet (follow-up PR).

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[reviewable-StarkWare]

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