Define Azure Blob capability matrix and implementation gaps

[asiffermann]

Context

Legacy issues #39 (PageBlobs) and #40 (Blob snapshots) both concern Azure Blob feature coverage. Reactivation consolidates them into a single capability-matrix effort.

Scope

• Define supported Azure Blob capabilities by feature area (block/page/append blobs, snapshots, and related operations)
• Identify current gaps in Proffer.Storage.Azure.Blobs
• Propose API and compatibility approach for missing capabilities
• Create implementation backlog slices from the matrix

Notes

Supersedes #39 and #40 in reactivation context.