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/parts_manifest_test.go

@@ -7,7 +7,7 @@ func TestParseRepoSpecNormalizesWestgateSSHHostToPublicGiteaHost(t *testing.T) {
 	if !ok {
 		t.Fatal("expected SSH remote to parse")
 	}
-	if spec.BaseURL != "https://gitea.westgate.pw" {
+	if spec.BaseURL != "https://git.westgate.pw" {
 		t.Fatalf("expected public Gitea base URL, got %q", spec.BaseURL)
 	}
 	if spec.Owner != "ShadowsOverWestgate" || spec.Repo != "sow-module" {