* fix(panel): narrow tooltip to hovered provider meter

Hovering one panel provider icon narrows the plasmoid tooltip to that
provider's quotas instead of listing the whole roster. Hovering elsewhere
keeps the full list. No stacked tooltip: the meter reports hover state and
the applet-owned tooltip renders the single line.

Docs: usage panel tooltip behavior; changelog Unreleased. TODO unaffected:
Plasma-native tooltip behavior, no CLI contract.

* fix(panel): drop stale hover when meter leaves rendered set

Visibility rules can filter a hovered meter out of compactProviders()
while its provider stays in the roster, and its destroyed MouseArea
delivers no reliable hover-exit. Narrow the tooltip only while the
hovered meter is actually rendered, and clear the stale hover when the
rendered meter set changes.

Addresses CodeRabbit review on PR #205.