fix(core): retry failed migrations

[ShiroKSH]

Summary

• Retry a migration after its callback fails instead of leaving it permanently marked complete.
• Claim the marker atomically, so concurrent callers do not run the same migration twice.
• Cover retry, completed-marker, and concurrent-call behavior with focused tests.

Root cause

The migration marker was written before the callback ran. A rejected callback left the marker in place, so later startups skipped an incomplete migration.

Checks

• npm run vitest -- util/paths.vitest.ts
• npm exec --yes --package=typescript@5.6.3 -- tsc -p ./ --noEmit
• npx --prefix core eslint core/util/paths.ts core/util/paths.vitest.ts
• npx prettier --check util/paths.ts util/paths.vitest.ts

[github-actions]

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[ShiroKSH]

I have read the CLA Document and I hereby sign the CLA