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.go

@@ -18,7 +18,7 @@ import (
 const (
 	defaultSowAssetsRepoOwner   = "ShadowsOverWestgate"
 	defaultSowAssetsRepoName    = "sow-assets"
-	defaultGiteaBaseURL         = "https://gitea.westgate.pw"
+	defaultGiteaBaseURL         = "https://git.westgate.pw"
 	partsManifestReleaseTag     = "parts-manifest-current"
 	partsManifestAssetName      = "sow-parts-manifest.json"
 	partsManifestCacheFileName  = "sow-parts-manifest.json"
@@ -148,7 +148,7 @@ func parseRepoSpec(raw string) (giteaRepoSpec, bool) {
 func normalizeGiteaHTTPHost(host string) string {
 	host = strings.TrimSpace(host)
 	if strings.HasPrefix(host, "git-ssh.") {
-		return "gitea." + strings.TrimPrefix(host, "git-ssh.")
+		return "git." + strings.TrimPrefix(host, "git-ssh.")
 	}
 	return host
 }