test: 覆盖 Form B 描述符的两处解析缺口

#355 需要 xpkg (.lua) 解析器读两个它此前**根本没读**的键,而两处都是静默的
——没读的键就只是「这个特性不存在」:

- `targets.<x>.required_features`:描述符因此无法表达让可选 host 工具变得
  可负担的成本门(compat.protobuf 的 protoc 会带进 libprotoc 的 ~157 个 TU,
  只要运行时的消费者绝不能编译它)。没有这个门,target 要么总是构建、
  要么根本拿不到。
- `deps` 的值此前只能是版本**字符串**,于是 Form B 描述符压根没有语法去向
  自己的依赖请求工具。

之前的 e2e 只走了 Form A(path 依赖)路径,这两处没有任何覆盖。第三个用例钉住
「未知 dep 键要被记录而不是吞掉」——半配置的依赖且毫无信号,正是既有的
per-feature 键记录机制要防的那种失败。

Author: speak-agent

tests/unit/test_xpkg_host_tools.cpp

@@ -0,0 +1,126 @@
+#include <gtest/gtest.h>
+
+import std;
+import mcpp.manifest;
+import mcpp.manifest.xpkg;
+import mcpp.platform;
+import mcpp.platform.axis;
+
+// #355 needed two things from the Form B (xpkg .lua) descriptor parser that it
+// simply did not read, and neither failed loudly:
+//
+//   * `targets.<x>.required_features` was dropped, so a descriptor could not
+//     express the cost gate that makes an optional host tool affordable —
+//     compat.protobuf's `protoc` pulls in libprotoc's ~157 extra TUs, which the
+//     consumers who only want the runtime must never compile. Without the gate
+//     the target is either always built or never available.
+//   * `deps` values could only be a version STRING, so a Form B descriptor had
+//     no syntax at all for requesting a tool from one of its own dependencies.
+//
+// Both were silent: an unread key is just an absent feature. These tests are
+// what makes them loud.
+
+namespace {
+
+mcpp::manifest::Manifest parse_or_fail(std::string_view lua) {
+    auto m = mcpp::manifest::synthesize_from_xpkg_lua(
+        lua, "compat.demo", "1.0.0", mcpp::platform::HostPlatform::current());
+    EXPECT_TRUE(m.has_value()) << (m ? "" : m.error().message);
+    return m.value_or(mcpp::manifest::Manifest{});
+}
+
+const mcpp::manifest::Target* find_target(const mcpp::manifest::Manifest& m,
+                                          std::string_view name) {
+    for (auto const& t : m.targets)
+        if (t.name == name) return &t;
+    return nullptr;
+}
+
+}  // namespace
+
+TEST(XpkgHostTools, TargetsCarryRequiredFeatures) {
+    auto m = parse_or_fail(R"LUA(
+package = {
+    spec = "1", name = "demo", namespace = "compat", type = "package",
+    mcpp = {
+        sources = { "*/src/**.cc" },
+        targets = {
+            ["demo"]   = { kind = "lib" },
+            ["protoc"] = { kind = "bin", main = "src/compiler/main.cc",
+                           required_features = { "protoc", "upb" } },
+        },
+    },
+}
+)LUA");
+
+    auto const* lib = find_target(m, "demo");
+    ASSERT_NE(lib, nullptr);
+    EXPECT_EQ(lib->kind, mcpp::manifest::Target::Library);
+    EXPECT_TRUE(lib->requiredFeatures.empty());
+
+    auto const* tool = find_target(m, "protoc");
+    ASSERT_NE(tool, nullptr);
+    EXPECT_EQ(tool->kind, mcpp::manifest::Target::Binary);
+    EXPECT_EQ(tool->main, "src/compiler/main.cc");
+    EXPECT_EQ(tool->requiredFeatures,
+              (std::vector<std::string>{"protoc", "upb"}));
+}
+
+TEST(XpkgHostTools, DepsAcceptBothAStringAndATable) {
+    // The string form is the long-standing one and must keep working
+    // unchanged; the table form is what lets a descriptor request a tool.
+    auto m = parse_or_fail(R"LUA(
+package = {
+    spec = "1", name = "demo", namespace = "compat", type = "package",
+    mcpp = {
+        sources = { "*/src/**.cc" },
+        deps = {
+            ["compat.zlib"]     = "1.3.2",
+            ["compat.protobuf"] = { version = "35.1", tools = { "protoc" } },
+        },
+    },
+}
+)LUA");
+
+    const mcpp::manifest::DependencySpec* zlib = nullptr;
+    const mcpp::manifest::DependencySpec* pb   = nullptr;
+    for (auto const& [k, spec] : m.dependencies) {
+        if (k.find("zlib") != std::string::npos)     zlib = &spec;
+        if (k.find("protobuf") != std::string::npos) pb   = &spec;
+    }
+
+    ASSERT_NE(zlib, nullptr);
+    EXPECT_EQ(zlib->version, "1.3.2");
+    EXPECT_TRUE(zlib->tools.empty());
+
+    ASSERT_NE(pb, nullptr);
+    EXPECT_EQ(pb->version, "35.1");
+    EXPECT_EQ(pb->tools, (std::vector<std::string>{"protoc"}));
+}
+
+TEST(XpkgHostTools, UnknownDepKeyIsRecordedRatherThanSwallowed) {
+    // A descriptor author writing an unsupported key must be told. Silently
+    // ignoring it leaves the dependency half-configured with no signal — the
+    // exact failure the per-feature key recording already exists to prevent.
+    auto m = parse_or_fail(R"LUA(
+package = {
+    spec = "1", name = "demo", namespace = "compat", type = "package",
+    mcpp = {
+        sources = { "*/src/**.cc" },
+        deps = {
+            ["compat.protobuf"] = { version = "35.1", no_such_key = "x" },
+        },
+    },
+}
+)LUA");
+
+    bool recorded = false;
+    for (auto const& k : m.xpkgUnknownKeys)
+        if (k.find("no_such_key") != std::string::npos) recorded = true;
+    EXPECT_TRUE(recorded) << "unknown dep key was swallowed";
+
+    // ...and the keys it DOES understand still take effect.
+    for (auto const& [k, spec] : m.dependencies)
+        if (k.find("protobuf") != std::string::npos)
+            EXPECT_EQ(spec.version, "35.1");
+}