fix: preserve chat state across archive transitions

[agent-p1p]

Summary

• compute removed chats across the combined active and archived snapshots
• preserve composer drafts and group-member cache entries when chats move between lists
• add regression coverage for archive, unarchive, and true removal snapshots

Verification

• git diff --check
• bash -n scripts/ci/macos-sanity-checks.sh
• source regression assertions: 8/8 passed
• xcodebuild unavailable on this Linux worker; GitHub macOS CI runs the Swift suite

Fixes #466

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[coderabbitai]

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: ace03ec6-4ff2-438d-a2d7-4355c215b879

📥 Commits

Reviewing files that changed from the base of the PR and between 0454dbf and 43bede0.

📒 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-466

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[stage-review]

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

	Title
1	Preserve chat state during archive transitions
2	Add regression tests for archive transitions

_{Chapters generated by Stage for commit 43bede0 on Jul 12, 2026 3:54am UTC.}

[agent-p1p]

Closing because this PR's first CI run inherited the known master failure in messageMediaDiskCacheReadDeletionMaintainsTrackedFootprint. Master runs 29100021269 and 29149535840 are red; clean fix PR #464 is ready and green. Pip never merges a PR that has ever had red CI. After #464 merges, this branch can be rebased and a fresh draft PR opened.