fix: let Attributes.Dig traverse nested Attributes values

[tas50]

Attributes.Dig asserted each level to map[string]any. A Go type assertion matches the dynamic type exactly, so it never matched an Attributes value — even though the two share an underlying type and convert freely.

Dig therefore only worked at the top level, where it converts explicitly (var cur any = map[string]any(a)). One level down:

n := &Node{Normal: Attributes{"network": Attributes{"gw": "10.0.0.1"}}}
n.Attribute("network.gw")   // → nil, false

JSON-decoded nodes were unaffected, because encoding/json only ever produces map[string]any — which is exactly why the existing tests missed it. But any attribute tree built in Go silently reported "not found", through Dig, GetString, Attribute and AttributeString alike.

Fixed by unwrapping both shapes in one helper.

Test plan

• TestDig_TraversesNestedAttributes — two levels of Attributes, via Dig and GetString
• TestNodeAttribute_TraversesNestedAttributes — via the Node precedence lookup
• Both failed before the change (Dig = <nil>, false)
• Existing JSON-decoded attribute tests still pass, so the map[string]any path is unchanged
• go vet ./... clean
• go test ./... -race -count=2
• No public surface change, so no README Status update needed