docs(packaging): add the 2.1.0 release entry

CI takes the newest <release> as the notes of a tag and refuses a tag
that does not match it. 2.1.0 covers what changed since 2.0.0: the
bundled LLVM 23.1.2, build outputs kept per compiler, and the std
module source chosen by the sysroot's libc++ release.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

packaging/flatpak/net.catcrafts.CrafterBuild.metainfo.xml

@@ -25,6 +25,12 @@
   </provides>
   <content_rating type="oars-1.1"/>
   <releases>
+    <release version="2.1.0" date="2026-10-03">
+      <description>
+        <p>The bundled toolchain is now LLVM 23.1.2. Build outputs are kept per compiler, so the first build after updating recompiles each project into a fresh directory and nothing has to be cleaned by hand.</p>
+        <p>Cross builds pick the std module source that matches the sysroot's libc++ release. The source for libc++ 22 ships beside the bundled 23 for sysroots that carry libc++ without one, such as Alpine, and a sysroot nothing matches fails with one line naming both releases. crafter-build --version prints the libc++ release in use.</p>
+      </description>
+    </release>
     <release version="2.0.0" date="2026-10-03">
       <description>
         <p>First release distributed as a Flatpak, with a bundled LLVM 22 toolchain and WASI sysroot. Windows host support is removed; crafter-build gains --version and resolves its compiler from one place.</p>