feat(observation): add stable observed-state fingerprints

[ElliotSun]

Summary

Add a platform-neutral, versioned semantic fingerprint for ObservedPlatformState so repeated observations can distinguish state changes from observation-envelope changes.

Semantics

The fingerprint identifies what platform state was observed, not where/when the observation happened.

Included in obs-v1:

• state platform;
• canonical asset identity;
• asset type;
• canonical property identity;
• physical type;
• nullability.

Excluded:

• source_identifier;
• captured_at;
• an existing fingerprint value;
• rich evidence not yet present in the minimal observation model.

Format:

obs-v1:sha256:<hex-digest>

Canonicalization

• canonical JSON with sorted keys;
• assets sorted independently of adapter/upstream ordering;
• properties sorted independently of adapter/upstream ordering;
• identity values use existing canonical_key semantics;
• provider-specific physical-type interpretation remains in provider adapters, not the fingerprint layer.

Changes

• add semapact.observation.fingerprint with canonical payload, hash, and immutable-copy helpers;
• populate the fingerprint on Databricks observations after mapping to the platform-neutral model;
• export the fingerprint API from semapact.observation;
• add a fixed golden v1 digest and determinism/change-sensitivity regression tests;
• update Databricks observation tests to require a populated stable fingerprint.

Boundary

Databricks TableInfo
        ↓
platform-neutral mapping
        ↓
ObservedPlatformState
        ↓
platform-neutral fingerprint
        ↓
fingerprinted ObservedPlatformState

No reconciliation, drift classification, contract import, governance, deployment, rich evidence, or lineage behavior is added here.