Use copy-on-write in driver.set to avoid ghost updates

[duelingbenjos]

Title: Use copy-on-write in driver.set to avoid ghost updates

Summary

• driver.set stores mutable values by reference in pending_writes/transaction_writes, so later in-place mutations by callers change the write intent and can leak stale/mutated data.

Problem

95:100:xian-contracting/src/contracting/storage/driver.py
if type(value) in [decimal.Decimal, float]:
    value = ContractingDecimal(str(value))
self.pending_writes[key] = value
if is_txn_write:
    self.transaction_writes[key] = value

• No defensive copy for dict/list.

Proposed Fix

• Deepcopy mutable inputs (dict/list) in driver.set before storing.
• Optionally store an immutable snapshot for transaction_writes used in API outputs.

Acceptance Criteria

• Mutating the original dict after set() does not affect what is committed or reported as writes.
• Unit tests demonstrate isolation of write intent from later mutations.