Add msgpack as alternative serialization format for entity storage

[deucalioncodes]

Problem

ic-python-db currently uses json.dumps/json.loads for all entity serialization in db_engine.py. While CPython 3.13 on WASI likely has the C-accelerated _json module, there is still overhead from:

1. JSON text format is larger than binary alternatives (string quoting, key repetition)
2. Every db.save() and db.load() goes through JSON encode/decode
3. On IC canisters, larger payloads mean more StableBTreeMap storage and more cycles

Proposal

Add msgpack as an alternative (or replacement) serialization format in db_engine.py.

Benefits

• Smaller payloads — msgpack is typically 30-50% smaller than JSON for the same data
• Faster encoding/decoding — even pure-Python msgpack is competitive with C-JSON for small dicts; a C-accelerated msgpack would be significantly faster
• Binary-native — no string escaping overhead for keys/values

Implementation approach

The change is localized to db_engine.py — swap json.dumps/json.loads with msgpack equivalents:

# db_engine.py
import msgpack  # or fallback to json

def save(self, type_name, id, data):
    key = f"{type_name}@{id}"
    self._db_storage.insert(key, msgpack.packb(data))
    
def load(self, type_name, id):
    key = f"{type_name}@{id}"
    data = self._db_storage.get(key)
    if data:
        return msgpack.unpackb(data)
    return None

Considerations

• Storage interface change: Storage.insert/get currently use str values. With msgpack the values would be bytes. Need to update Storage ABC and MemoryStorage.
• Migration: Existing canister data is JSON-encoded. Need a migration path or dual-format support (try msgpack first, fall back to JSON).
• WASI C extension: For maximum benefit, the msgpack C extension would need to be cross-compiled for wasm32-wasip1 and linked into the CPython WASM binary. Pure-Python msgpack may not outperform C-accelerated JSON.
• Audit log: Also uses json.dumps — should be updated consistently.
• Depends on: Issue Fix redundant save on every Entity.load() — eliminates unnecessary json.dumps + storage write #1 (redundant save on load) should be fixed first to reduce the number of serialization calls before optimizing the serialization itself.