fix(storage): keep graph reads available during Windows cleanup contention

[rabii-chaarani]

Summary

• Defer retired-generation deletion on Windows when an open database file denies removal, while keeping cleanup_pending visible and retrying later.
• Restore the retirement marker after a partial delete so the next cleanup still finds the generation.
• Add a Windows regression that holds the retired database open, verifies a newly active generation accepts search calls while cleanup is pending, then verifies collection after the handle closes.

CI failure

Run 36081364568, Windows native artifact job failed because graph_search returned Access is denied. (os error 5) during refresh recovery.

Verification

• cargo fmt --check
• git diff --check
• cargo test -p codebase-graph --locked --test generation_storage (5 passed on macOS)
• cargo test -p codebase-graph --locked --test refresh_recovery_process blocked_refresh_recovers_after_memory_budget_restore_without_source_edit (1 passed on macOS with loopback access)

The PR CI run passed all checks. Its Windows job passed both managed_reads_survive_deferred_retired_generation_deletion and the originally failing blocked_refresh_recovers_after_memory_budget_restore_without_source_edit test.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-25T02:17:16.823448Z	25889e3	PR opened

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