Add cutover concurrent test

[morgo]

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Further notes in https://github.com/block/spirit/blob/main/.github/CONTRIBUTING.md

This is a pretty isolated/easy way to test the cutover better. It is inspired by one of the tests added to MoveTables.

Also fixes #548 again, since there was a second issue here. It ensures that the buffered position is always going forward. There is a case where FormatDescriptionEvent events could prevent BlockWait from completing.

[Copilot]

Pull request overview

This PR adds a concurrent test for cutover atomicity to verify that the migration system correctly maintains consistency between the old and new tables during the cutover process. The test uses an intentionally partial cutover that only renames the original table to _old (skipping the rename of _new to original), creating a failure scenario that allows verification of data consistency under concurrent write load.

• Adds a test-only cutover mechanism (useTestCutover) that performs a partial rename for failure injection testing
• Implements TestCutoverAtomicityWithConcurrentWrites to verify consistency between _old and _new tables with concurrent writes
• Refactors cutover logic to extract executeRenameUnderLock as a shared implementation for both normal and test cutovers

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
pkg/migration/migration.go	Adds private useTestCutover field to control test-only cutover behavior
pkg/migration/runner.go	Passes useTestCutover flag from Migration to cutover configuration
pkg/migration/cutover.go	Adds partialRenameForTest method and refactors rename logic into shared executeRenameUnderLock function
pkg/migration/cutover_test.go	Adds comprehensive concurrent test with helper functions to simulate write load during migration

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