fix(new): resolve namespaced packages in --template fetch

`mcpp new --template llmapi` failed with `fetch 'llmapi@0.2.6' failed`:
the index lookup finds pkgs/l/llmapi.lua by bare filename, but the
namespace stayed "" while the descriptor declares namespace="mcpplibs"
(legacy name="mcpplibs.llmapi") — and xlings resolves install targets
by the descriptor's qualified name.

fetch_template_package now derives the structured (namespace, shortName)
from the descriptor fields via the new pm::compat::descriptor_coordinates
helper instead of trusting the filename hit. Root-index packages (imgui)
keep namespace "" and still install by bare name; legacy embedded/dotted
names and canonical namespace fields all normalize to the same
coordinates. Covered by unit tests in test_pm_compat.

Author: Sunrisepeak

src/cli.cppm

@@ -1068,11 +1068,9 @@ fetch_template_package(const mcpp::scaffold::TemplateSpec& spec) {
 
     // Namespace candidates mirror dependency lookup: index root first,
     // then the compat namespace.
-    std::string ns;
     std::optional<std::string> lua;
     for (std::string cand : {std::string{}, std::string{"compat"}}) {
         if (auto l = fetcher.read_xpkg_lua(cand, spec.pkg)) {
-            ns = cand;
             lua = std::move(*l);
             break;
         }
@@ -1083,16 +1081,28 @@ fetch_template_package(const mcpp::scaffold::TemplateSpec& spec) {
             "(check the name, or run `mcpp index update`)", spec.pkg));
     }
 
+    // The filename hit alone does not carry the namespace: a bare spec
+    // like "llmapi" finds pkgs/l/llmapi.lua even though the descriptor
+    // declares `namespace = "mcpplibs"`, and xlings resolves install
+    // targets by the descriptor's qualified name. Derive the structured
+    // (namespace, shortName) from the descriptor fields.
+    auto coords = mcpp::pm::compat::descriptor_coordinates(
+        spec.pkg,
+        mcpp::manifest::extract_xpkg_namespace(*lua),
+        mcpp::manifest::extract_xpkg_name(*lua));
+    const std::string& ns        = coords.namespace_;
+    const std::string& shortName = coords.shortName;
+
     std::string version = spec.version;
     if (version.empty()) {
-        auto v = mcpp::pm::resolve_semver(ns, spec.pkg, "*", fetcher);
+        auto v = mcpp::pm::resolve_semver(ns, shortName, "*", fetcher);
         if (!v) return std::unexpected(v.error());
         version = *v;
     }
 
-    auto installed = fetcher.install_path(ns, spec.pkg, version);
+    auto installed = fetcher.install_path(ns, shortName, version);
     if (!installed) {
-        auto fq = ns.empty() ? spec.pkg : std::format("{}.{}", ns, spec.pkg);
+        auto fq = ns.empty() ? shortName : std::format("{}.{}", ns, shortName);
         mcpp::ui::info("Downloading", std::format("{} v{}", fq, version));
         CliInstallProgress progress;
         std::vector<std::string> targets{ std::format("{}@{}", fq, version) };
@@ -1101,7 +1111,7 @@ fetch_template_package(const mcpp::scaffold::TemplateSpec& spec) {
             "fetch '{}@{}': {}", fq, version, r.error().message));
         if (r->exitCode != 0) return std::unexpected(std::format(
             "fetch '{}@{}' failed (exit {})", fq, version, r->exitCode));
-        installed = fetcher.install_path(ns, spec.pkg, version);
+        installed = fetcher.install_path(ns, shortName, version);
         if (!installed) return std::unexpected(std::format(
             "package '{}@{}' install path missing after fetch", fq, version));
     }
@@ -1123,7 +1133,7 @@ fetch_template_package(const mcpp::scaffold::TemplateSpec& spec) {
         return std::unexpected(std::format(
             "package '{}@{}' has no mcpp.toml", spec.pkg, version));
     }
-    return FetchedTemplatePackage{root, spec.pkg, version};
+    return FetchedTemplatePackage{root, shortName, version};
 }
 
 void print_template_listing(const FetchedTemplatePackage& pkg,

src/pm/compat.cppm

@@ -73,6 +73,47 @@ inline ResolvedName resolve_package_name(std::string_view name,
     return r;
 }
 
+// ─── Descriptor-derived coordinates ──────────────────────────────────
+//
+// Derive the structured (namespace, shortName) for a package whose index
+// descriptor was located by filename candidates from an unqualified spec
+// (`mcpp new --template <pkg>`). The filename hit alone is not enough:
+// pkgs/l/llmapi.lua is found by the bare name "llmapi" while the
+// descriptor declares `namespace = "mcpplibs"` (and possibly a legacy
+// dotted `name = "mcpplibs.llmapi"`), and xlings resolves install targets
+// by the descriptor's qualified name.
+//
+// Unlike resolve_package_name(), a bare name with no namespace field
+// stays in the index root (namespace "") — root packages such as "imgui"
+// install by their bare name.
+
+inline ResolvedName descriptor_coordinates(std::string_view specPkg,
+                                           std::string_view luaNs,
+                                           std::string_view luaName)
+{
+    ResolvedName r;
+    std::string name(luaName.empty() ? specPkg : luaName);
+    r.namespace_ = std::string(luaNs);
+
+    if (!r.namespace_.empty()) {
+        // Legacy descriptors embed the namespace in the name too
+        // (namespace = "mcpplibs", name = "mcpplibs.llmapi").
+        auto prefix = r.namespace_ + ".";
+        if (name.starts_with(prefix)) {
+            name = name.substr(prefix.size());
+            r.usedLegacySplit = true;
+        }
+    } else if (auto dot = name.find('.'); dot != std::string::npos) {
+        // Legacy dotted name without a namespace field.
+        r.namespace_ = name.substr(0, dot);
+        name         = name.substr(dot + 1);
+        r.usedLegacySplit = true;
+    }
+
+    r.shortName = std::move(name);
+    return r;
+}
+
 // Reconstruct the fully-qualified name from (namespace, shortName).
 // Default-namespace packages use the bare short name; others use
 // "ns.short".

tests/unit/test_pm_compat.cpp

@@ -93,3 +93,58 @@ TEST(DependencySelector, ExplicitRootSelectorHasOnlyThatRoot) {
     EXPECT_EQ(selector.candidates[0].namespace_, "compat");
     EXPECT_EQ(selector.candidates[0].shortName, "gtest");
 }
+
+// ─── descriptor_coordinates (package-template fetch) ────────────────
+
+TEST(PmCompat, DescriptorCoordinatesLegacyEmbeddedNamespace) {
+    // pkgs/l/llmapi.lua: namespace = "mcpplibs", name = "mcpplibs.llmapi"
+    auto r = mcpp::pm::compat::descriptor_coordinates(
+        "llmapi", "mcpplibs", "mcpplibs.llmapi");
+
+    EXPECT_EQ(r.namespace_, "mcpplibs");
+    EXPECT_EQ(r.shortName, "llmapi");
+    EXPECT_TRUE(r.usedLegacySplit);
+}
+
+TEST(PmCompat, DescriptorCoordinatesCanonicalNamespaceField) {
+    auto r = mcpp::pm::compat::descriptor_coordinates(
+        "llmapi", "mcpplibs", "llmapi");
+
+    EXPECT_EQ(r.namespace_, "mcpplibs");
+    EXPECT_EQ(r.shortName, "llmapi");
+    EXPECT_FALSE(r.usedLegacySplit);
+}
+
+TEST(PmCompat, DescriptorCoordinatesRootPackageStaysInRoot) {
+    // pkgs/i/imgui.lua: namespace = "", name = "imgui" — must NOT be
+    // promoted to the default namespace (it installs by its bare name).
+    auto r = mcpp::pm::compat::descriptor_coordinates("imgui", "", "imgui");
+
+    EXPECT_EQ(r.namespace_, "");
+    EXPECT_EQ(r.shortName, "imgui");
+    EXPECT_FALSE(r.usedLegacySplit);
+}
+
+TEST(PmCompat, DescriptorCoordinatesLegacyDottedNameWithoutNamespace) {
+    auto r = mcpp::pm::compat::descriptor_coordinates(
+        "tinyhttps", "", "mcpplibs.tinyhttps");
+
+    EXPECT_EQ(r.namespace_, "mcpplibs");
+    EXPECT_EQ(r.shortName, "tinyhttps");
+    EXPECT_TRUE(r.usedLegacySplit);
+}
+
+TEST(PmCompat, DescriptorCoordinatesFallsBackToSpecWhenNameMissing) {
+    auto r = mcpp::pm::compat::descriptor_coordinates("imgui", "", "");
+
+    EXPECT_EQ(r.namespace_, "");
+    EXPECT_EQ(r.shortName, "imgui");
+}
+
+TEST(PmCompat, DescriptorCoordinatesCompatNamespace) {
+    auto r = mcpp::pm::compat::descriptor_coordinates(
+        "mbedtls", "compat", "compat.mbedtls");
+
+    EXPECT_EQ(r.namespace_, "compat");
+    EXPECT_EQ(r.shortName, "mbedtls");
+}