Panel signals passthrough has no drift guard

[FlyM1ss]

Found reviewing #117.

get_latest_panel_payload returns the wire's signals` dict raw:

signals = body.get("signals") or {}
...
"signals": signals,

Nothing projects it, so _alarm_if_all_dropped — which can only ever see what a projection dropped — is blind to it. It guards the two feed projections only.

Nothing server-side reads sentiment_score. The browser does:

Number(s.sentiment_score).toFixed(2)   // -> "NaN" if the key is missing

So a producer rename of sentiment_score renders NVDA NaN in every chip with no drop, no exception, no degraded badge, no ERROR log. The feed keeps building (it reads headline/url/source/published), so the drift alarm cannot fire. That is the 2026-07-14 title/link outage one field over, and the score -> sentiment_score rename passed within inches of it.

Pre-existing; not introduced by #117. Fixed in #120.