apollo_starknet_os_program,starknet_os: add the Cairo0 proof-fact leaf digest

[einat-starkware]

Third PR of the privacy proof-fact fold stack. First Cairo0 piece: proof_fact_fold.cairo with the leaf layer only — compute_leaf_output_digest (felt encoding + blake opcode) and the circuit-hash getters (dw data tables, the stdlib _get_sigma idiom), plus the standalone-program build wiring and the Rust↔Cairo test harness (an entry-point runner for the module's functions). Not yet reachable from the OS program.

Every Cairo function is tested for bit-exact agreement with the Rust mirror, including felt-encoding boundary cases around 2^63.

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

🤖 Generated with Claude Code

https://claude.ai/code/session_01XmPJM3Wph4QLmFmhcxVsh4

[cursor]

PR Summary

Low Risk
Test-only Cairo module and parity tests; no OS runtime or production registry wiring yet.

Overview
Introduces the first Cairo0 piece of the privacy proof-fact fold stack: a standalone proof_fact_fold module that mirrors the proving side’s leaf layer so digests can later be checked against the verifier’s public output.

proof_fact_fold.cairo adds compute_leaf_output_digest (encode felts after the two version markers, then BLAKE2s via opcode) and getters for pinned leaf-verifier and multiverifier circuit hashes (dw tables). The module is not linked into the main OS program yet.

Build and tests: compile_test_contracts compiles this Cairo file to proof_fact_fold_bytes, exposed as PROOF_FACT_FOLD_BYTES. proof_fact_fold_test gains a small Cairo entry-point runner and rstest cases that assert bit-exact agreement with the existing Rust proof_fact_fold mirror, including felt-encoding boundaries around 2^63.

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[reviewable-StarkWare]

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