fix(interp): eval_decl handles FnExtern (#328 build-failure root cause)

The `TopFn fd` arm of `eval_decl` matched `fd.fd_body` against only
`FnBlock` and `FnExpr`, leaving `FnExtern` unmatched. Every prior interp
test imported externs via `use effects::{…}` so the case never fired;
the bug was latent.

The STDLIB-04a (#328) hermetic tests are the first to include an inline
`extern fn make_ref<T>(x: T) -> Ref<T> / Mut;` declaration in the source
they hand to `Interp.eval_program`. That triggers the missing arm and
raises "Pattern matching failed" at `lib/interp.ml:1010`, which is the
"2 failures!" reported by `dune runtest` on every PR since #334.

Fix: wrap the body lowering in an outer `match fd.fd_body with`. The
`FnExtern` arm returns `Ok env` (externs are runtime-bound via
`create_initial_env`'s `VBuiltin` table — `panic`/`error`/`make_ref`/
`get`/`set`/etc. — not via the AST). The inner `FnBlock|FnExpr` arm
keeps the existing closure construction; an `FnExtern` fall-through is
unreachable per the outer guard and asserted to make that explicit to
the reader.

This unbreaks main: the 2 failing tests
(`#328 make_ref/set/get round-trip (Int)` and `(String)`) now have a
defined evaluation path. The codegen-only test of the same trio
already passed (it bypasses interp).

Refs #328.

Author: claude

lib/interp.ml

@@ -1027,14 +1027,25 @@ let create_initial_env () : env =
 let eval_decl (env : env) (decl : top_level) : env result =
   match decl with
   | TopFn fd ->
-    let closure = VClosure {
-      cl_params = fd.fd_params;
-      cl_body = (match fd.fd_body with
-        | FnBlock blk -> ExprBlock blk
-        | FnExpr e -> e);
-      cl_env = env;
-    } in
-    Ok (extend_env fd.fd_name.name closure env)
+    (match fd.fd_body with
+     | FnExtern ->
+       (* Externs have no AST body to evaluate. Their runtime binding is
+          provided by [create_initial_env]'s builtin table (panic, error,
+          make_ref, …). Skip here so an inline `extern fn` declaration in
+          a test source (e.g. the STDLIB-04a Mut round-trip tests) doesn't
+          blow up the [FnBlock|FnExpr] match below.
+          Refs #328 root-cause for the interp pattern-match-failure. *)
+       Ok env
+     | FnBlock _ | FnExpr _ ->
+       let closure = VClosure {
+         cl_params = fd.fd_params;
+         cl_body = (match fd.fd_body with
+           | FnBlock blk -> ExprBlock blk
+           | FnExpr e -> e
+           | FnExtern -> assert false (* unreachable: outer match guards *));
+         cl_env = env;
+       } in
+       Ok (extend_env fd.fd_name.name closure env))
 
   | TopConst tc ->
     let* v = eval env tc.tc_value in