refactor(core): centralize SHA-256 digest ownership

[morluto]

Fixes #741.

Problem

The generic byte-to-sha256: digest helper lived in storage.identity, forcing unrelated contracts and persistence code either to repeat it or to take an inappropriate storage dependency. That location creates both an import-cycle risk and an ownership error: byte hashing is canonicalization infrastructure, not storage behavior.

Change

Move the helper to dependency-free jacobian.canonical and migrate the SAT, SMT, linear, and storage-blob callers to that owner. The output remains exactly sha256: plus the lowercase SHA-256 hexadecimal digest.

storage.identity no longer exports the helper. This is intentional: retaining a re-export would leave the obsolete dependency path and compatibility shim in place.

Validation

• make test-unit TESTS=tests/unit/contracts/test_canonical_json.py tests/unit/contracts/test_sat_contracts.py tests/unit/contracts/test_smt_contracts.py tests/unit/contracts/test_linear_contracts.py — 40 passed
• make test-storage TESTS=tests/boundary/storage/transactions/test_artifact_integrity.py — 16 passed
• make check-static — passed

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