feat: grey the dashboard dot for undeployed nodes - #143

A fleet dashboard node with no engine deployed drew the same green dot as a node that is running and ready, so a grid of undeployed nodes read as healthy at a glance when it is actually empty.

Summary

• Add a fifth dashboard health tier, not serving, for nodes that answered their last refresh but are not serving anything: engine state idle or stopped, with no start or stop in flight.
• Its status dot is the faded grey the unknown tier already uses, but a filled dot rather than the ?: a node known to be undeployed stays distinct from one the dashboard has not heard from yet.
• Update the fleet-client health-indicator spec from four tiers to five, and move the byte-stable idle tile and the tier table test to the new tier, with stopped and start-in-flight rows added.

Implementation details

• stopped is covered alongside idle: a daemon whose engine was stopped and a remote environment at scale to zero report the same state, and two off states wearing two different dots on one grid would read as inconsistent.
• The tier case sits after the empty-state check and immediately before default in dashHealthTierFor, so the existing precedence is untouched: a start in flight over an idle report still reads attention, a failed outcome still reads unhealthy.
• Only the dot changes: the tile body keeps printing exactly what fleet metrics prints, and no other surface draws the glyph.