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diff --git a/source/util/Diag.mc b/source/util/Diag.mc
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+// Author:  cirakg & claude ai
+// Summary: Debug-only diagnostics (start-up time, memory) for `make perf` and `make memtest`.
+//
+// Debug builds print one machine-readable line per event to the simulator
+// console: "DIAG", the event name, then key=value pairs, for example
+//     DIAG first_frame startup_ms=1 mem_used=6984 mem_total=781888
+// tools/check_budgets.py reads these lines and compares them with
+// tools/budgets.json. Plain key=value pairs are used instead of JSON because
+// the simulator console escapes quotes. Release builds compile the empty
+// (:release) versions instead, so a real watch running a release build never
+// logs anything.
+//
+// Security rule: only timings and memory counters ever go through here, never
+// secrets, codes or entry names. This is the only file allowed to call
+// System.println outside tests (enforced by tools/lint_project.py).
+
+import Toybox.Lang;
+import Toybox.System;
+
+module Diag {
+
+    // System.getTimer() value when the app object was created.
+    (:debug)
+    var startMs as Number = 0;
+
+    // Start the start-up timer. Called from OtpWalletApp.initialize().
+    (:debug)
+    function markStart() as Void {
+        startMs = System.getTimer();
+    }
+
+    (:release)
+    function markStart() as Void {
+    }
+
+    // Report start-up time and memory use after the first complete frame.
+    // startup_ms covers app object creation to the end of the first draw. The
+    // time the system needs to load the program before that is invisible to
+    // the app; app-diag.sh measures it from outside (README section 18).
+    (:debug)
+    function firstFrame() as Void {
+        var stats = System.getSystemStats();
+        System.println("DIAG first_frame startup_ms=" + (System.getTimer() - startMs)
+            + " mem_used=" + stats.usedMemory + " mem_total=" + stats.totalMemory);
+    }
+
+    (:release)
+    function firstFrame() as Void {
+    }
+}