feat(blob): add optional affinity field to /blob/add arguments

[Copilot]

Per fil-one/RFC#15, adds an optional affinity field to /blob/add so callers (e.g. Ingot) can constrain Sprue's storage-node selection to a specific set of Piri DIDs without imposing static space-level routing state.

Changes

• commands/blob/types.go: AddArguments gains Affinity []did.DID (omitempty). nil → field absent, existing routing unchanged. Non-nil → encoded as a list of storage-node DIDs.
• commands/blob/cbor_gen.go, json_gen.go: Regenerated via go generate.

// Pin a write to a specific Piri node
args := &blob.AddArguments{
    Blob:     myBlob,
    Affinity: []did.DID{localPiriDID},
}

// Omit affinity → Sprue's default routing applies (backward-compatible)
args := &blob.AddArguments{Blob: myBlob}