Add standalone Nexus operation API boilerplate

Register all 8 RPCs (Describe, List, Start, Poll, Count, Cancel,
Terminate, Delete) with stub implementations returning Unimplemented.
Add FrontendHandler interface and wire it into the frontend service.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: stephanos

chasm/lib/nexusoperation/frontend.go

@@ -0,0 +1,27 @@
+package nexusoperation
+
+import (
+	"go.temporal.io/server/common/log"
+	"go.temporal.io/server/common/namespace"
+)
+
+// FrontendHandler provides the frontend-facing API for standalone Nexus operations.
+type FrontendHandler interface{}
+
+type frontendHandler struct {
+	config            *Config
+	logger            log.Logger
+	namespaceRegistry namespace.Registry
+}
+
+func NewFrontendHandler(
+	config *Config,
+	logger log.Logger,
+	namespaceRegistry namespace.Registry,
+) FrontendHandler {
+	return &frontendHandler{
+		config:            config,
+		logger:            logger,
+		namespaceRegistry: namespaceRegistry,
+	}
+}

chasm/lib/nexusoperation/fx.go

@@ -19,6 +19,11 @@ var Module = fx.Module(
 	fx.Invoke(register),
 )
 
+var FrontendModule = fx.Module(
+	"chasm.lib.nexusoperations.frontend",
+	fx.Provide(NewFrontendHandler),
+)
+
 func register(
 	registry *chasm.Registry,
 	library *Library,