fix: bind Binance reconciliation to configured account

[Pigbibi]

Summary

• require the expected reconciliation digest bundle to include the account-scope hash
• compare broker account identity against that configured binding instead of the observation itself
• keep all reconciliation reads and execution controls unchanged

Validation

• .venv/bin/python -m pytest -q (253 passed, 20 subtests)
• focused reconciliation tests (11 passed)
• git diff --check

Scope

No broker write, baseline enrollment, runtime activation, or order path is added.