feat(server): persist node semantic context and retain CAD agent context

Add a semantic_context column to canvas_nodes (schema, sqlc queries,
generated models, repository upserts) and thread SemanticContext through the
domain node, API type, and mappers so CAD-derived context survives canvas
saves.

Treat agent text parts with textSource "cad-context" like settings: keep
them for planning, memory, and image-generation prompts but exclude them
from the persisted user-visible message copy. Add coverage for the CAD
context visibility and mapper behavior.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/logic/cad_context_mapper_test.go

@@ -0,0 +1,22 @@
+package logic
+
+import (
+	"testing"
+
+	"img_infinite_canvas/internal/domain/design"
+)
+
+func TestCanvasNodeSemanticContextRoundTrip(t *testing.T) {
+	nodes := []design.Node{{
+		ID:              "cad-1",
+		Type:            design.NodeTypeImage,
+		Title:           "plan.svg",
+		LayerRole:       "cad-vector",
+		SemanticContext: "CAD semantic context: layer ROOMS",
+	}}
+
+	roundTrip := toDomainNodes(toAPINodes(nodes))
+	if len(roundTrip) != 1 || roundTrip[0].SemanticContext != nodes[0].SemanticContext {
+		t.Fatalf("expected semantic context round trip, got %#v", roundTrip)
+	}
+}