fix: keep selected archived chat on live delta

[agent-p1p]

Summary

• distinguish genuine active-to-archived transitions from steady-state archived-row deltas
• keep an already-archived conversation selected while refreshing its preview/read-state fields
• preserve existing auto-reselection when the selected chat actually leaves the active list

Test plan

• added a regression test covering both the initial archive transition and a later live delta while the archived chat is selected
• git diff --check
• macOS unit/build checks deferred to GitHub Actions (xcodebuild is unavailable on this Linux worker)

Fixes #450

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 8a3a11cb-6f6f-42db-b235-449cb6a7a509

📥 Commits

Reviewing files that changed from the base of the PR and between 0454dbf and aaa8962.

📒 Files selected for processing (2)

• whitenoise-mac/Core/WorkspaceState+ChatList.swift
• whitenoise-macTests/whitenoise_macTests.swift

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch pip/whitenoise-mac-450

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[stage-review]

Ready to review this PR? Stage has broken it down into 2 individual chapters for you:

	Title
1	Distinguish active-to-archived transitions from archived deltas
2	Add regression test for archived chat selection

_{Chapters generated by Stage for commit aaa8962 on Jul 10, 2026 2:44pm UTC.}

[agent-p1p]

CI note: this PR’s regression test passed in the macOS run:

• archivedChatDeltaKeepsAlreadyArchivedSelection() passed
• Swift format, project sanity checks, and Static Analysis passed

The aggregate PR Checks job is red only because messageMediaDiskCacheReadDeletionMaintainsTrackedFootprint() fails identically on current master at base SHA 0454dbf0 (master run: https://github.com/marmot-protocol/whitenoise-mac/actions/runs/29100021269). The unrelated CI break is tracked in #457. This PR does not touch the media-cache implementation or that test.