Fix source bundle export with locked Registry dependencies

Locked resolution does not necessarily populate the Registry index cache.
Source bundle generation requires these indexes but unconditionally uses
offline Registry access, causing `lito fetch --locked --output` to fail
from a clean cache.

Allow source bundle generation to fetch missing Registry indexes when the
fetch command is online, while retaining offline behavior for offline
fetches.
--- ../driver/src/command/fetch.cpp	2026-09-26 13:13:57.701710134 +0300
+++ ../driver/src/command/fetch.cpp	2026-09-26 13:16:57.862695687 +0300
@@ -338,6 +338,7 @@
                          const Vec<lito::tools::acquisition::VerifiedFile>&  archive_files,
                          const CargoFetchOutcome&                            cargo,
                          bool                                                cargo_offline,
+                         bool                                                registry_offline,
                          const Option<config::LitoBootstrapConfig>&          registries,
                          lito::tools::ToolResolver&                          resolver,
                          const ResolvedProcessEnvironment&                   environment,
@@ -461,6 +462,22 @@
     auto registry_packages = rstd::collections::BTreeMap<String, empty>::make();
     auto registry_indices  = rstd::collections::BTreeMap<String, empty>::make();
     auto registry_cache    = Option<PathBuf> {};
+    auto registry_http =
+        Option<Box<lito::registry::CurlRegistryHttpTransport>> {};
+    if (! registry_offline) {
+        auto curl = resolver.require(
+            lito::tools::Tool::Curl,
+            lito::tools::command_tool_requirement(
+                lito::tools::HostToolCapability::HttpDownload,
+                "Registry source bundle export"_str));
+        if (curl.is_err()) {
+            cleanup();
+            return Err(rstd::into<CommandError>(rstd::move(curl).unwrap_err()));
+        }
+        registry_http =
+            Some(Box<lito::registry::CurlRegistryHttpTransport>::make(
+                curl->executable.clone(), environment));
+    }
     for (const auto& source : graph.sources) {
         if (source.kind != lito::source::PackageSourceKind::Registry) continue;
         if (builtin_package_source(graph, source)) continue;
@@ -487,12 +504,17 @@
         auto index_destination = layout.registry_index(pin.release.package);
         auto index_key         = index_destination.as_path().to_string_lossy();
         if (! registry_indices.contains_key(index_key.as_str())) {
+            auto http = registry_http.is_some()
+                            ? (**registry_http).transport()
+                            : lito::registry::RegistryHttpTransport {};
             auto indices = lito::registry::RegistryIndexClient(
                 registry_cache->clone(),
                 **configured,
-                lito::registry::RegistryNetworkPolicy::Offline,
+                registry_offline
+                    ? lito::registry::RegistryNetworkPolicy::Offline
+                    : lito::registry::RegistryNetworkPolicy::Online,
                 lito::registry::RegistryIndexUpdatePolicy::Reuse,
-                {});
+                http);
             auto record = indices.source_bundle_record(pin);
             if (record.is_err()) {
                 cleanup();
@@ -645,6 +667,8 @@
                                                archive_files,
                                                cargo,
                                                request.cargo.offline,
+                                               request.sources.network ==
+                                                   lito::source::NetworkPolicy::Offline,
                                                request.registries,
                                                resolver,
                                                *environment,