CmkKpiStatCard: replace the delta with a stale note

A stale reading (series ending in missing samples) still showed a
live-looking delta. Detect staleness from a trailing null run and
show a muted "No recent data - last sample HH:MM" note in its slot
instead, on its own line so it doesn't overflow.

Adds a "Stale" playground option alongside Gap.

CMK-38083

Change-Id: Ib2f0ea204b48a23b8108ea74769a4123c5720dd4