Placement compatibility expansion

[JSYoo5B]

Summary

• Adds Placement-compatible APIs for resource providers, inventories, traits, aggregates, usage, allocations, allocation candidates, resource classes, and trait catalog behavior.
• Splits placement service methods by resource and groups service repositories.

Main Review Points

• Placement resource and inventory modeling.
• Allocation and candidate response shape.
• Service/repository grouping after the Placement surface grows.

Verification

• go test ./...
• Checked each commit from this slice onward with go test ./...

[JSYoo5B]

Placement has enough repositories that grouping them into a struct improves service construction readability. This is the same explicit dependency approach as compute, just packaged for a wider surface.

[JSYoo5B]

Allocation candidates are intentionally computed from current providers, inventories, and usages rather than stored as their own resource. This is the key semantic endpoint in the Placement slice.

[JSYoo5B]

Capacity follows the simple Placement formula used by this MVP: total minus reserved, multiplied by allocation ratio, then reduced by usage. Nested providers or sharing providers would need a more advanced model later.