refactor(compat): preserve legacy import paths

[Pavkazzz]

Summary

Stack 5/7 — compatibility shims for the 0.10 refactor.

• Preserve imports from the former top-level driver modules.
• Preserve legacy balancer-policy import paths.
• Preserve the hasql.psycopg3.PoolAcquireContext identity alias.
• Replace the transitional legacy SQLAlchemy adapter with the canonical shim and remove its now-inapplicable regression.
• Keep compatibility intentionally narrow: removed raw manager state proxies remain an explicitly documented 0.10 breaking change.

Stack

Base: #39 (refactor/4-drivers)
Next: #31

Verification

• Backward-import and alias regressions passed in the affected local matrix.
• Ruff and mypy passed.
• Required GitHub CI: lint, mypy, and Python 3.10–3.14 PostgreSQL test matrix.