feat(check): reject borrow-returns at the signature (#55, 0121 task 1)

`(ret (borrow T))` is now a check error (CheckError::BorrowReturnNotPermitted,
code `borrow-return-not-permitted`), fired on the fn signature before
body dataflow. Borrow-returns are refcount-invisible and can outlive
their source; re-enabling them needs the escape/liveness axis, which is
out of scope (spec 0062). The diagnostic names that gap as an honest
"not yet".

The check_fn signature destructure now also binds `ret_mode` (param_modes
stays under `..`; the borrow-over-value reject that reads it is folded
into the cutover, task 4). RED-first: examples/borrow_return_reject.ail
checked clean (exit 0) before the reject landed, exits 1 after. New
CLI-boundary E2E pin crates/ail/tests/mode_signature_rejects.rs, modelled
on raw_buf_new_type_arg_pin.rs. Additive — ParamMode::Implicit still
present; full workspace suite green, bench/check.py 0 regressed.

refs #55

examples/borrow_return_reject.ail

@@ -0,0 +1,14 @@
+(module borrow_return_reject
+
+  (data Box
+    (doc "Heap cell holding one Int.")
+    (ctor Box (con Int)))
+
+  (fn peek
+    (doc "MUST FAIL post-cutover: borrow-return is refcount-invisible and can outlive its source; re-enabling needs the escape axis.")
+    (type
+      (fn-type
+        (params (own (con Box)))
+        (ret (borrow (con Box)))))
+    (params b)
+    (body b)))