Phase 3: move service layer, serializers and DTOs into WarpEngine + dummy-app test suite

- all catalog services (update/software/highlighted/builds/file/file-manager/
  download/image + SoftwareResponseBuilder), the SoftwareUpdater platform
  services and their concerns, Blueprinter serializers (incl. TimestampFields)
  and the 4 DTOs now live in the engine under WarpEngine::
- constantize dispatch strings use absolute names
  (WarpEngine::SoftwareUpdater::<Platform>Service)
- container paths read from WarpEngine.config everywhere (FileService,
  DownloadService, FileManagerService, ArchiveExtraction, ReleaseSerializer
  path rewriting); FileManagerService base path is now lazy
- engine requires blueprinter itself; gemspec declares blueprinter + rubyzip
- engine test suite: spec/dummy app (mysql warp_engine_test, catalog-only
  schema), rails_helper with engine-local factories; catalog model/service
  specs and factories moved from the host
- host suite keeps TTG specs and loads catalog factories from the engine

Verified: engine suite 40 green, host suite 14 green, /api/software and
/api/builds byte-identical to baselines, admin OK.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

spec/models/platform_link_spec.rb

@@ -0,0 +1,52 @@
+require "rails_helper"
+
+RSpec.describe WarpEngine::PlatformLink, type: :model do
+  describe "validations" do
+    it { should validate_presence_of(:name) }
+    it { should validate_presence_of(:url) }
+    it { should validate_presence_of(:platform) }
+
+    it "rejects invalid platform" do
+      link = build(:platform_link, platform: "invalid")
+      expect(link).not_to be_valid
+      expect(link.errors[:platform]).to be_present
+    end
+
+    it "accepts valid platforms" do
+      WarpEngine::PlatformLink::SUPPORTED_PLATFORMS.each do |p|
+        link = build(:platform_link, platform: p)
+        expect(link).to be_valid
+      end
+    end
+  end
+
+  describe "default scope" do
+    it "excludes soft-deleted records" do
+      active = create(:platform_link)
+      create(:platform_link, deleted_at: Time.current)
+
+      expect(WarpEngine::PlatformLink.all).to eq([active])
+    end
+
+    it "orders by position" do
+      second = create(:platform_link, position: 2)
+      first  = create(:platform_link, position: 1)
+
+      expect(WarpEngine::PlatformLink.all).to eq([first, second])
+    end
+  end
+
+  describe ".for_platform" do
+    it "returns links for given platform only" do
+      tic80_link = create(:platform_link, platform: "tic80")
+      create(:platform_link, platform: "love")
+
+      result = WarpEngine::PlatformLink.for_platform("tic80")
+      expect(result).to eq([tic80_link])
+    end
+
+    it "returns empty array for platform without links" do
+      expect(WarpEngine::PlatformLink.for_platform("godot")).to eq([])
+    end
+  end
+end