fix: copy fallback uses legacy heuristic to validate copied content

Round-6 review fix:

After Round-5 made is_install_complete() strict marker-only, the copy
fallback path broke:

    bool copyOk = copy_xpkg_from_global(verdir);
    if (copyOk && is_install_complete(verdir)) {   // ← always false
        mark_install_complete(...);                //   never reached
        return make_payload();
    }
    clean_incomplete_install(verdir);              // ← wipes the copy

copy_xpkg_from_global() doesn't (and can't) write .mcpp_ok in the
copied directory, so the marker-only check would always fail, and the
just-copied package would be immediately wiped, returning "xpkg payload
missing".

Fix: validate the copied content via looks_complete_legacy() (the
structural heuristic) before writing the marker. This is safe in this
context because:

  1. Step 2 of the resolve chain already cleaned any pre-existing
     residue using strict marker-only semantics — so anything at
     verdir now MUST be the result of our just-completed copy.

  2. copy_xpkg_from_global() only returns true on a clean recursive
     copy (no partial copies reach this branch).

  3. The heuristic validates that the source actually had content
     (rules out copying from an empty/broken global xlings dir).

This restores the documented "copy_xpkg_from_global is the typical fast
fallback" behavior that Round-5 unintentionally broke.

Author: Sunrisepeak

src/pm/package_fetcher.cppm

@@ -703,13 +703,20 @@ Fetcher::resolve_xpkg_path(std::string_view target,
     }
 
     // 4. Copy fallback: xlings may have installed into its global data dir.
+    //    copy_xpkg_from_global() only returns true on a clean recursive
+    //    copy, but it doesn't write .mcpp_ok (the source predates this
+    //    feature). Validate content via the legacy layout heuristic, then
+    //    write the marker ourselves. We use looks_complete_legacy() here
+    //    rather than is_install_complete() (marker-only) because we just
+    //    copied this directory in: any "interrupted install" residue would
+    //    have been removed at step 2.
     bool copyOk = mcpp::fallback::copy_xpkg_from_global(verdir);
-    if (copyOk && mcpp::fallback::is_install_complete(verdir)) {
+    if (copyOk && mcpp::fallback::looks_complete_legacy(verdir)) {
         mcpp::fallback::mark_install_complete(verdir);
         mcpp::log::verbose("fetcher", "resolved via copy fallback");
         return make_payload();
     }
-    // Copy failed or incomplete — clean partial copy.
+    // Copy failed or produced an unrecognizable layout — clean partial copy.
     mcpp::fallback::clean_incomplete_install(verdir);
 
     // 5. All paths exhausted — return the most informative error.