Make Sources health fail closed

[FZ2000]

Effect in one sentence

Makes Sources health fail closed: a current ingestion error always outranks retained success, so old healthy rows and watcher state cannot look current.

State contract

Retained snapshot	Current error	Presentation
No	No	Loading
Yes	No	Connected source/watcher details
Either	Yes	Source health unavailable; retained rows hidden

What changes

• SourcesPresentationState gives the current request result precedence.
• DashboardStore.refreshIngestion() owns an independent refresh/error/freshness lane.
• Full refresh still starts lanes concurrently; Sources retry does not refresh receipts, plan, or usage.
• Cancellation is not published as a source-health failure.
• The top bar shows source-specific freshness while Sources is active.
• Four deterministic light/dark minimum/reference images cover retained-data failure without leaking local paths.

Review order

1. Sources presentation state and semantic tests.
2. Dashboard store/top-bar refresh ownership.
3. Sources unavailable state, explanation, and Retry.
4. Sources harness and canonical references.

Verification at current head 2085312

• Refreshed onto current main (4ab2547); the only conflict combined Setup and Sources in the documented visual-suite inventory.
• Current-head GitHub CI passes pytest on Python 3.11/3.12/3.13 and the macOS app/snapshot job.
• swift test — 124 executed, 7 expected opt-in visual skips, 0 failures.
• Canonical SetupVisualRegressionTests and SourcesVisualRegressionTests — all references match locally on the pinned renderer.
• swift build -c release — passed.
• git diff --check origin/main...HEAD — clean.

Scope

This changes source-health presentation and refresh isolation only. The pre-existing minimum-width truncation of long verifier descriptions remains out of scope.