GeezerVison

Fixes #232.

Two SDK instances can both observe a pending account-database migration before either applies it. Their subsequent transactions then duplicate the same schema change (or insert duplicate migration markers). Acquire SQLite's write reservation with BEGIN IMMEDIATE and recheck update_done inside that transaction before applying commands. The migration and its completion marker still commit or roll back together.

The regression synchronizes two independent connections after both observe the pending migration, covering versions 1–6 and checking that stored account data survives. All six cases fail without the fix. A separate test verifies rollback after invalid SQL and a successful retry. Includes a changelog fragment.

Validation:

• Linux Python 3.12.13 full unit/doctest suites: v3 776 passed / 63 skipped; v2 783 / 66; v1 858 / 97; v0 701 / 374.
• Serial v3 coverage run: 776 passed, 63 skipped, 76% overall coverage. An earlier parallel coverage run stalled and was interrupted; the serial run completed.
• Ruff check across source/tests/noxfile, format check across source/tests, static test and dependency-license check passed. The untouched noxfile.py has an existing format-check failure.
• Windows Python 3.13: 748 passed, 79 skipped, 12 symlink tests failed with WinError 1314. The same 12 fail on the unchanged production code; all migration regressions pass with the fix.
• No live B2 integration or macOS validation performed.

Prepared and tested with Codex assistance.