[Feature] Allow SimpleStorage placement to target eligible Ray nodes #150

Problem

Since #144, SimpleStorage hard round-robins storage units across every alive Ray node. This preserves even distribution on a homogeneous cluster, but it also places storage units across unrelated node groups in a shared heterogeneous cluster.

A public two-node reproduction is:

# Node A
ray start --head --resources='{"storage_pool": 1}'

# Node B
ray start --address='<head-address>' --resources='{"compute_pool": 1}'

With both nodes alive, initializing SimpleStorage schedules units across both nodes even when only nodes advertising storage_pool should host storage.

Proposed behavior

Add an optional SimpleStorage placement filter based on a Ray custom resource, for example an eligible custom-resource name or equivalent node filter.

• When the option is unset, keep the current behavior exactly: round-robin across all alive Ray nodes.
• When set, include only alive nodes whose matching custom resource has a positive capacity.
• Keep hard NodeAffinitySchedulingStrategy placement and round-robin only within the eligible set.
• If no alive node matches, fail during initialization with a clear error instead of creating actors that cannot satisfy the intended placement.
• Dead nodes must never be eligible.

This should only filter the node list used by SimpleStorage; it should not monkeypatch Ray scheduling or change other storage backends.

Suggested tests

• Unset option preserves all-alive-node round-robin behavior.
• A custom-resource filter excludes otherwise alive nodes.
• Dead nodes are excluded even when they advertise the resource.
• No matching alive node raises a clear initialization error.