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/application/cad_context_test.go

@@ -0,0 +1,43 @@
+package application
+
+import (
+	"strings"
+	"testing"
+
+	"img_infinite_canvas/internal/domain/design"
+)
+
+func TestCADContextIsModelInputButNotVisibleUserCopy(t *testing.T) {
+	messages := []design.AgentChatMessage{
+		{
+			Role: "user",
+			Contents: []design.AgentContent{
+				{Type: "text", Text: "Focus on the living room", TextSource: "input"},
+				{Type: "image", ImageURL: "/canvas-assets/floor-plan.webp", ImageName: "floor-plan crop.webp"},
+				{Type: "text", Text: "CAD semantic context: layer ROOMS, block ROOM_A", TextSource: "cad-context"},
+			},
+		},
+	}
+
+	modelPrompt := promptFromAgentMessages(messages)
+	visiblePrompt := visiblePromptFromAgentMessages(messages)
+	if !strings.Contains(modelPrompt, "layer ROOMS") {
+		t.Fatalf("expected model prompt to retain CAD semantics, got %q", modelPrompt)
+	}
+	if strings.Contains(visiblePrompt, "layer ROOMS") {
+		t.Fatalf("expected visible user copy to hide CAD semantics, got %q", visiblePrompt)
+	}
+	if !strings.Contains(visiblePrompt, "Focus on the living room") || !strings.Contains(visiblePrompt, "floor-plan crop.webp") {
+		t.Fatalf("expected visible prompt and reference capsule directive, got %q", visiblePrompt)
+	}
+}
+
+func TestCADContextAloneDoesNotSatisfyPromptValidation(t *testing.T) {
+	messages := []design.AgentChatMessage{{
+		Role:     "user",
+		Contents: []design.AgentContent{{Type: "text", Text: "CAD semantic context", TextSource: "cad-context"}},
+	}}
+	if hasUserPromptTextInAgentMessages(messages) {
+		t.Fatal("expected hidden CAD context not to count as user prompt text")
+	}
+}