test: keep Baseline fault injection effective on Windows

[dayebishouji]

What changes and why

On Windows, blockBaselineBackupReplacement does not reliably cause a Baseline backup failure: AtomicWrite can rename the nonempty backup directory aside and successfully publish the file. This makes failure/recovery tests unexpectedly succeed or fail while removing the displaced blocker.

Keep the directory open until the fixture is unblocked, preventing the Windows rename-aside fallback. Register the idempotent unblock function with t.Cleanup so tests that discard the returned function also close the handle. Existing failure and recovery assertions are unchanged.

Refs #79, specifically the Windows test-fixture report. This is an independent test-only follow-up; the console-window fix remains in #80.

Affected public contracts

• No public contract changes.

No production code, CLI/MCP behavior, data format, or transaction identity changes.

Verification

• Native Windows, Go 1.26.8: go test ./internal/cli -count=1 fails on unchanged a24fb8b (15 top-level tests; 17 failure events including subtests), then passes with this patch (236.525s). Top-level totals: 451 pass / 15 fail / 1 skip before; 466 pass / 0 fail / 1 skip after.
• Focused Apply, recovery, and update-entry cases were also reproduced before the change and pass afterward.
• Linux make fast.
• Linux make verify completed all gates. Every make full constituent passed, including go test ./... (CLI 105.708s), race tests (CLI 667.546s), staticcheck, licensing, govulncheck, and clean-room smoke. The integration-tag database tests ran without configured live engines and therefore do not provide real-engine acceptance.
• MCP conformance: 46 passed, 0 failed.
• MCP scenarios: 61 passed, 0 characterized, 0 failed.
• Lifecycle: 53 passed, 2 characterized, 1 failed. The database suite could not start Docker because Docker is unavailable in this WSL distribution; all other lifecycle groups completed. Consequently, the overall make verify command is not green (exit 2).
• Remote fast-ci: all 8 checks passed for 7105b4c.
• git diff --check and independent code review.

Operating system impact

Windows tests retain an open directory handle to prevent its replacement. POSIX already refuses replacing a directory with a file; the same helper remains portable. The handle is closed before fixture cleanup.

Migration and recovery

No migration or runtime recovery behavior changes.

Cognition

• Completed AOCI maintenance, Observe acknowledgement, Verify, Check, and terminal Guide; governance is aligned.

The ordinary package test stays in Observe. Only its Baseline fingerprint is refreshed; no cognition Entry changes.