test: sibling projects inherit the loading project's args

ProjectArgs loads a chain app -> lib -> leaf of LocalProjects with --debug
and a project flag. lib, loaded without .args, has to see both, and so
does its own sibling, which takes the default one level further down.
Copies loaded with `.args = {}` and with an explicit list get nothing and
exactly that list. The GitProjectSpec default is checked on the spec
itself, since a GitProject would need a repository to clone. Without the
defaults, five of its checks fail.

ProjectDependency now also loads its consumer with a project flag: the
pragma dependency must still come back with no args. Without the explicit
`.args = {}` in the pragma path it gets the consumer's flag.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/ProjectArgs/fixture/lib/project.cpp

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+import Crafter.Build;
+using namespace Crafter;
+
+// The middle of the chain. Its own sibling takes the default too, so
+// whatever this file was loaded with has to reach the leaf unchanged.
+extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> args) {
+    Configuration* leaf = LocalProject({ .projectFile = "../leaf/project.cpp" });
+
+    Configuration cfg;
+    cfg.path = "./";
+    cfg.name = "lib";
+    cfg.outputName = "lib";
+    cfg.type = ConfigurationType::LibraryStatic;
+    ApplyStandardArgs(cfg, args);
+    cfg.dependencies = { leaf };
+    return cfg;
+}