debug: RED-pin for instance-body unbound-var bug

ail check returns ok for a program that references an unbound
identifier inside an instance method's lambda body. The error
surfaces later at ail build as a degraded mono-pass diagnostic
(monomorphise_workspace: unknown identifier) without source
location, symbol kind, or "did you mean" candidates.

Cause: check_def at crates/ailang-check/src/lib.rs:1593 early-
returns Ok(()) for Def::Instance without walking method bodies.
The arm's comment promises instance-body typechecking from iter
22b.2; that wiring was never implemented. Only the workspace-
load coherence checks (Orphan/Duplicate/MissingMethod) touch
instance defs, and they inspect schema only, not the method-
body identifier graph.

This commit lands RED only. GREEN follows via implement mini-
mode.

Fixture: examples/bug_unbound_in_instance_method.ail
RED test: crates/ail/tests/unbound_in_instance_method_pin.rs
  - check_fires_unbound_var_for_str_concat_in_instance_method_body
  - check_json_unbound_var_in_instance_method_body

examples/bug_unbound_in_instance_method.ail

@@ -0,0 +1,18 @@
+(module bug_unbound_in_instance_method
+  (class Describe
+    (param a)
+    (method describe
+      (type (fn-type (params (borrow a)) (ret (con Str))))))
+  (instance
+    (class Describe)
+    (type (con Int))
+    (method describe
+      (body
+        (lam
+          (params (typed n (con Int)))
+          (ret (con Str))
+          (body (app str_concat "n=" (app int_to_str n)))))))
+  (fn main
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body (do io/print_str (app describe 5)))))