fix(topdata): target git.westgate.pw and stub released parts manifest in tests

The reprovision retired gitea.westgate.pw in favour of git.westgate.pw, but
internal/topdata still defaulted to the old host and made a live API call
during the native buildability check, so once CI checkout was fixed the
parts tests failed (DNS, then HTTP 404).

- defaultGiteaBaseURL -> https://git.westgate.pw; normalize git-ssh. -> git.
- Add stubEmptyPartsManifest (stub_test.go) and call it from the 2 non-optional
  parts tests so they exercise the build offline. Per-test (t.Setenv) on
  purpose: a global TestMain SOW_ASSETS_SERVER_URL would override the
  git-remote-derived server other tests configure. VFX tests already pass
  (optional consumer ignores HTTP 404).

make check + make smoke green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/topdata/topdata_test.go

@@ -2499,6 +2499,7 @@ func TestBuildReferenceAndCompareReference(t *testing.T) {
 
 func TestCompareReferenceHandlesMixedModeOutputListMismatch(t *testing.T) {
 	root := testProjectRoot(t)
+	stubEmptyPartsManifest(t)
 	mkdirAll(t, filepath.Join(root, "topdata", "data", "parts"))
 	writeFile(t, filepath.Join(root, "topdata", "base_dialog.json"), "{}\n")
 	writeFile(t, filepath.Join(root, "topdata", "data", "parts", "belt.json"), `{
@@ -2834,6 +2835,7 @@ func TestBuildAppliesNonTLKOverrides(t *testing.T) {
 
 func TestBuildFallsBackWhenProjectMigrationIsIncomplete(t *testing.T) {
 	root := testProjectRoot(t)
+	stubEmptyPartsManifest(t)
 	mkdirAll(t, filepath.Join(root, "topdata", "data", "parts"))
 	writeFile(t, filepath.Join(root, "topdata", "base_dialog.json"), "{}\n")
 	writeFile(t, filepath.Join(root, "topdata", "data", "parts", "belt.json"), `{
@@ -13470,7 +13472,7 @@ func TestBuildNativeRejectsGitRepoTopDataAssetsOverride(t *testing.T) {
 	writeFile(t, filepath.Join(root, "reference", "build.py"), "print('ok')\n")
 
 	proj := testProject(root)
-	proj.Config.TopData.Assets = "ssh://git@gitea.westgate.pw:2222/ShadowsOverWestgate/sow-assets.git"
+	proj.Config.TopData.Assets = "ssh://git@git.westgate.pw:2222/ShadowsOverWestgate/sow-assets.git"
 
 	_, err := BuildNative(proj, nil)
 	if err == nil || !strings.Contains(err.Error(), "no longer supports git repo specs") {