Make CompactedStorage construction logically empty

[PraneethMerugu]

Summary

• make the qualified CompactedStorage constructor establish the same empty invariant as Collection => Allocate()
• centralize cold KA initialization under compacted storage ownership
• delegate collection allocation to the canonical constructor
• cover grouped, projected, zero-capacity, ownership, and non-aliasing behavior

Validation

• LocalMath Pkg.test() — 1340/1340 passed
• focused real-Metal authoring witness — 27/27 passed on Julia 1.12.6
• focused storage and collection tests passed