Add canonical state digest for integrity verification

[rlippmann]

Summary

Add a stable cryptographic digest for authoritative Engine state so a committed state snapshot has an explicit integrity identity independent of Python's built-in hash() semantics.

Motivation

Python's __hash__() / hash() is intended for hash-table behavior, is not cryptographic, and is not stable across interpreter runs for string-containing values. Context Compiler needs a separate concept if state integrity or stable snapshot identity is to be checked explicitly.

A canonical digest would complement, not replace, immutable state representation.

Architectural direction

• Derive the digest from a canonical representation of authoritative state.
• Use a cryptographic digest such as SHA-256 rather than Python's built-in object hash.
• Exclude the digest value itself from the material being digested.
• Make the digest deterministic across processes/runs for the same logical state.
• Compute/update it only when constructing or committing a new authoritative snapshot.
• Keep the current public state behavior and serialization contract stable unless an explicit persistence-format decision says otherwise.
• Add tests proving equal canonical states produce equal digests and meaningful state changes produce different digests.

Uses

• integrity verification / detection of unexpected state mutation;
• stable state identity for diagnostics or checkpoints;
• groundwork for later optimistic-concurrency checks.

Boundary

A digest is a detection/identity mechanism, not a same-process security boundary. Code capable of arbitrarily modifying both authoritative state and its stored digest can still bypass it.

Deferred / separate

• immutable authoritative-state representation: Harden authoritative Engine state against direct mutation #278;
• locking, compare-and-swap, retry, or other concurrency semantics;
• whether a digest is exported/persisted as part of a future state schema;
• native/process isolation for hostile-code threat models.