fix: skip redundant save on Entity.load()

[deucalioncodes]

Summary

Every Entity.load() was triggering a full re-serialization and write-back via __init__ → _save(), even though the data was just read from storage.

Before (per load)

storage.get → json.loads → __init__ → _save() → serialize() → json.dumps → storage.insert

• 1× json.loads (read)
• 1× serialize() (Python dict construction, MRO walk)
• 1× json.dumps (redundant write-back)
• 1× storage.insert (redundant StableBTreeMap write on canister)
• 1× audit json.dumps (if audit enabled)

After (per load)

storage.get → json.loads → __init__ → done

• 1× json.loads (read)

Changes

1. __init__: Skip _save() when _loaded=True — the data is already in storage.
2. Entity.load(): Explicitly call _save() after migration, so migrated data is still persisted.

Test results

106 passed, 3 pre-existing failures (unrelated TestDatabase.setUp issue when run via pytest).

Closes #1