docs(interface): document InterfaceMesh and InterfaceDOFHandler scope

Outline coupled surface grids and placeholder roadmap beyond Seg2 layouts.

src/interface/README.md

@@ -0,0 +1,17 @@
+# src/interface/
+
+First-class interface geometry and DOF numbering for coupled problems (mortar
+contact, FSI traces, interface reactions, surface chemistry), kept separate from
+volume `Mesh` / `DOFHandler`.
+
+Types:
+
+- `AbstractInterfaceMesh`, `InterfaceMesh` — embedded polygonal or segment grids
+  (`Seg2` first) plus per-segment `InterfaceVolumeCoupling` to slave/master volume
+  elements.
+- `InterfaceDOFHandler`, `create_interface_elements!` — same assignment discipline as
+  volume handlers (`Vertex` / `Cell` on the interface mesh only; no `Edge`/`Face`
+  until facet maps on interfaces exist).
+
+Mortar quadrature, projection kernels, and composition with volume operators are
+implemented on top of these structures in future work.