arc: honour a values file's required node exclusions in placement with burst and in the measured ceiling #145

A values file can keep runner pods off some nodes with a required node affinity, for example template.spec.affinity.nodeAffinity.requiredDuringSchedulingIgnoredDuringExecution: {nodeSelectorTerms: [{matchExpressions: [{key: node-role.kubernetes.io/control-plane, operator: DoesNotExist}]}]} to keep CI jobs off control-plane nodes. The role does not know about it in two places.

arc_measured_max_runners counts every node matching the eligibility label and the profile's node_selector, so a node the required term excludes still adds to maxRunners. The scale set then creates runner pods that can only sit Pending until a slot on another node frees.

With burst set, arc_runner_placement writes its own requiredDuringSchedulingIgnoredDuringExecution.nodeSelectorTerms (eligible node or burst node), and the overlay's recursive combine replaces the values file's list, so the exclusion is dropped without a message. This is the required-term twin of #143.

Repro for the first: an eligible node labelled node-role.kubernetes.io/control-plane=true with room for one runner, and the values file above. The measured ceiling includes that node's runner.

A profile key such as node_exclusions: [<label key>, ...] (or required match expressions in general) that arc_runner_placement adds to every required term it writes, and that arc_measured_max_runners applies when it picks the nodes it counts, would cover both.