fix: attribute media-only chat previews

[agent-p1p]

Fixes #471

Summary

• Compute the media-only Attachment fallback before sender attribution instead of returning early.
• Apply the existing incoming-message sender prefix to attachment previews.
• Update regression coverage so incoming media-only previews with a display name render as Alice: Attachment.

Verification

• git diff --check — passed.
• bash -n scripts/ci/macos-sanity-checks.sh — passed.
• scripts/ci/macos-sanity-checks.sh — expected local failure on Linux: xcodebuild: command not found.
• CI-exact xcrun swift-format, Xcode unit tests, release build, and static analysis require macOS and will run in GitHub Actions.

Sensitive paths

none

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: b9bcf215-1e76-4c1d-a5e5-7b84a90e6633

📥 Commits

Reviewing files that changed from the base of the PR and between 0454dbf and bcb944f.

📒 Files selected for processing (2)

• whitenoise-mac/Core/MarmotMapping.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-471

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[stage-review]

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

	Title
1	Attribute media-only previews to senders
2	Update regression tests for media previews

_{Chapters generated by Stage for commit bcb944f on Jul 12, 2026 3:37am UTC.}

[agent-p1p]

Pip adversarial review

Verdict: merge-blocked by required CI; no defect found in the PR's source change.

Blocking

1. Required PR Checks is red. Mac CI run 29178495489 failed the 418-test plan in messageMediaDiskCacheReadDeletionMaintainsTrackedFootprint() (three assertions at lines 2224/2254/2267), so this PR is ineligible for merge under Pip's ever-red policy. The same deterministic failure is already present on master at base SHA 0454dbf and tracked by #457; it is unrelated to this two-file preview change. Restore a green base via CI: restore the macOS PR test plan after media-cache changes #457, then retire/recreate this PR rather than merging or merely rerunning an ever-red PR.

Diff review

• The media-only fallback is now assigned to body and flows through the exact existing chat-bubble sender-attribution guard. Incoming previews with a nonblank display name become Alice: Attachment; outgoing previews, missing/blank names, and non-chat presentations retain their prior unprefixed behavior.
• Literal Unsupported message text remains distinguishable because isMediaOnlyChat still requires empty source plaintext.
• Scope is minimal: 11 insertions / 6 deletions across the mapper and existing regression test; no sensitive path touched.
• The changed regression mediaOnlyChatPreviewShowsAttachmentLabelInsteadOfUnsupported() passed on the PR head. Static Analysis, Swift format lint, and macOS sanity checks also passed. git diff --check origin/master...HEAD passed locally.
• CodeRabbit did not perform a review because its quota was exhausted; its status context is not substantive review evidence.

Counts

• Blocking: 1 (required CI / ever-red policy; known base failure CI: restore the macOS PR test plan after media-cache changes #457)
• Suggestions: 0
• Nitpicks: 0

[agent-p1p]

Retiring this PR under Pip’s ever-red policy. The required PR Checks failure is the known master regression tracked by #457, not this diff. PR #464 is green and ready to restore the base; once it merges, this branch will be rebased and a fresh PR created so the replacement has no inherited red CI history.