🐛 Add recursion depth limit to Fressian reader (#11020)

Bound read depth at 128 levels to prevent StackOverflowError from
crafted deeply-nested payloads. All recursive read handlers go
through read-object!, so a single depth check covers all paths.

AI-assisted-by: mimo-v2.5-pro

common/src/app/common/fressian.clj

@@ -31,6 +31,11 @@
   ([^String s, ^String encoding]
    (.getBytes s encoding)))
 
+;; --- DEPTH TRACKING
+
+(def ^:dynamic *read-depth* 0)
+(def ^:const max-read-depth 128)
+
 ;; --- LOW LEVEL FRESSIAN API
 
 (defn write-object!
@@ -41,7 +46,13 @@
 
 (defn read-object!
   [^Reader r]
-  (.readObject r))
+  (when (>= *read-depth* max-read-depth)
+    (throw (ex-info "maximum Fressian read depth exceeded"
+                    {:type :validation
+                     :code :max-read-depth-reached
+                     :hint "maximum Fressian read depth exceeded"})))
+  (binding [*read-depth* (inc *read-depth*)]
+    (.readObject r)))
 
 (defn write-tag!
   ([^Writer w ^String n]

common/test/common_tests/fressian_test.clj

@@ -21,7 +21,8 @@
   (:import
    java.time.Instant
    java.time.OffsetDateTime
-   java.time.ZoneOffset))
+   java.time.ZoneOffset
+   java.util.UUID))
 
 ;; ---------------------------------------------------------------------------
 ;; Helpers
@@ -524,3 +525,18 @@
     (t/is (d/ordered-map? rt))
     (t/is (= om rt))
     (t/is (= (keys om) (keys rt)))))
+
+(t/deftest decode-rejects-excessive-recursion-depth
+  ;; N2-01: deeply nested structures must be rejected before stack overflow
+  (let [depth  (+ fres/max-read-depth 50)
+        data   (reduce (fn [acc _i] [acc])
+                       :leaf
+                       (range depth))
+        encoded (fres/encode data)]
+    (try
+      (fres/decode encoded)
+      (t/is false "expected exception for excessive recursion depth")
+      (catch clojure.lang.ExceptionInfo e
+        (let [d (ex-data e)]
+          (t/is (= :validation (:type d)))
+          (t/is (= :max-read-depth-reached (:code d))))))))