Add ProjectID to stream event envelope for project channel (CF3)

Why: D50 approved — the project inbox needs its own SSE stream reusing the
conversation event mechanism via the pseudo-key project:<projectId>; the
envelope stamps project ownership (additive field).
Evidence: conformance golden event_project_stream; go test green.
Impact: additive only (omitempty); consumers ignore unknown fields.
Refs: documents proposal 2026-09-30-card-stream-artifact-unify-project-inbox.md (D50).

testdata/golden/event_project_stream.json

@@ -0,0 +1,19 @@
+{
+  "seq": 47,
+  "conversationId": "project:prj_abc123def456",
+  "projectID": "prj_abc123def456",
+  "ts": "2026-09-30T11:00:00Z",
+  "v": 1,
+  "type": "card.created",
+  "payload": {
+    "card": {
+      "id": "card_def456abc789",
+      "projectId": "prj_abc123def456",
+      "kind": "result",
+      "title": "项目级卡",
+      "status": "done",
+      "createdAt": "2026-09-30T11:00:00Z",
+      "updatedAt": "2026-09-30T11:00:00Z"
+    }
+  }
+}