fix: make FPS telemetry opt-in

docs/internal/upstream/pr13-fps-telemetry.md

@@ -3,7 +3,9 @@
 Parked from NyperYuhgard's PSXrecomp PR #13, commit
 [`b49548e7b8f494c9584eb09039a8c325d7375b46`](https://github.com/mstan/psxrecomp/commit/b49548e7b8f494c9584eb09039a8c325d7375b46).
 
-This branch retains only the title-neutral simulated-vblank FPS and realtime-speed
-telemetry. It was adapted to current master and guards window-title access in
-headless/windowless operation. The overlay-cache inventory from the same upstream
-commit is parked separately.
+This branch retains only the simulated-vblank FPS and realtime-speed telemetry.
+It was adapted to current master, guards window-title access in
+headless/windowless operation, and is disabled by default. Set
+`PSX_FPS_TELEMETRY=1` before launch to opt in to the title-bar counter and
+matching stderr line. The overlay-cache inventory from the same upstream commit
+is parked separately.

runtime/src/main.cpp

@@ -355,6 +355,16 @@ static int      s_netplay_depth24_present_skip = 0;
 static uint32_t s_fmv_skip_last_mdec = 0;
 static int      s_fmv_skip_hold = 0;
 
+/* Lightweight title/stderr FPS telemetry is developer-facing and opt-in.
+ * Keep release window titles clean unless explicitly requested. */
+static bool fps_telemetry_enabled() {
+    static const bool enabled = [] {
+        const char* value = std::getenv("PSX_FPS_TELEMETRY");
+        return value && value[0] && value[0] != '0';
+    }();
+    return enabled;
+}
+
 static void smooth_60_reset(void) {
     g_smooth_60_state.previous_source.clear();
     g_smooth_60_state.source_hash = 0;
@@ -3334,14 +3344,14 @@ static void sdl_vblank_present(void) {
      * than presents so turbo and skipped-frame modes still report game speed.
      * Skip during netplay post-load barrier — admit is stalled and the window
      * is not updating, so a climbing FPS line is misleading. */
-    if (!psx_netplay_in_load_barrier()) {
+    if (fps_telemetry_enabled() && !psx_netplay_in_load_barrier()) {
         extern uint64_t s_frame_count;
         const Uint64 now = SDL_GetPerformanceCounter();
         const Uint64 frequency = SDL_GetPerformanceFrequency();
         if (!s_fps_last_time) {
             s_fps_last_time = now;
             s_fps_last_frame = s_frame_count;
-            if (sdl_window) {
+            if (sdl_window && s_fps_base_title.empty()) {
                 const char *title = SDL_GetWindowTitle(sdl_window);
                 if (title) s_fps_base_title = title;
             }

runtime/tests/test_fps_telemetry_opt_in.py

@@ -0,0 +1,20 @@
+#!/usr/bin/env python3
+"""Structural guard: FPS title/stderr telemetry stays explicitly opt-in."""
+
+from pathlib import Path
+
+
+main_cpp = (Path(__file__).parents[1] / "src" / "main.cpp").read_text(
+    encoding="utf-8"
+)
+
+required = (
+    'std::getenv("PSX_FPS_TELEMETRY")',
+    "fps_telemetry_enabled() && !psx_netplay_in_load_barrier()",
+    "sdl_window && s_fps_base_title.empty()",
+)
+for needle in required:
+    if needle not in main_cpp:
+        raise AssertionError(f"missing FPS telemetry opt-in guard: {needle}")
+
+print("PASS: FPS telemetry remains opt-in and title-stable")