feat(store): source-bucket from CapabilityEntry.source_kind

Studio's `_sourceForItem` now prefers the wire-level
`sourceKind` field (populated by 0.12+ hubs) over the legacy
name-prefix heuristic. Adds a "temporal" bucket alongside
"mcp" / "n8n" / "native" / "federated" so SPARK-federated
workflows render with their own transport pill on the store
grid + filter chip.

Pre-0.12 hubs return empty `sourceKind` and the legacy
sniffing path still works -- back-compat preserved.

`StoreItem` data class accepts an optional `sourceKind`
parameter (defaults to empty). dart analyze green.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/data/hub.dart

@@ -597,6 +597,7 @@ class HubService {
             docsUrl: e.docsUrl,
             kind: e.kind,
             provider: e.provider,
+            sourceKind: e.sourceKind,
           ),
         )
         .toList();
@@ -1399,6 +1400,12 @@ class StoreItem {
   /// Provider id for federated entries. Empty for native.
   /// E.g. "filesystem" for an `mcp.filesystem.read_file` tool.
   final String provider;
+  /// Transport delivering this entry. Distinct from `kind` —
+  /// `sourceKind` answers "HOW does the hub reach this", not
+  /// "is it installable". Values: "bundle", "mcp", "n8n",
+  /// "temporal", "webhook". Empty falls back to deriving
+  /// from `kind` for pre-0.12 hubs.
+  final String sourceKind;
 
   bool get isFederated => kind == 'federated';
 
@@ -1423,5 +1430,6 @@ class StoreItem {
     required this.docsUrl,
     required this.kind,
     required this.provider,
+    this.sourceKind = '',
   });
 }