Expose asset timestamp probe boundary

docs/modernization/debt.md

@@ -242,6 +242,11 @@ agent or engineer to remove them without reconstructing context from chat.
   `plan_asset_file_load` wrapper still performs the retained `stat` probe for
   mtime platforms until asset/file watching is owned by injected storage
   services.
+- 2026-06-12: DEBT-0054 was narrowed again. The retained mtime probe is now an
+  explicit `pp_platform_api::probe_asset_file_timestamp` boundary with missing
+  and existing file coverage, and `plan_asset_file_load_for_platform` consumes
+  that probe instead of owning the `stat` call inline. Injected file-watch or
+  storage services still need to own the live probe before the debt closes.
 - 2026-06-05: DEBT-0056 was narrowed. `src/asset.h` no longer exposes Android
   SDK types or forward declarations; retained Android asset-manager and asset
   handles are stored as opaque pointers and cast only inside `src/asset.cpp`,

docs/modernization/roadmap.md

@@ -644,6 +644,8 @@ mobile platform keyboard bridges continue.
 `pano_cli plan-cursor-visibility` exposes the app-core cursor visibility
 decision used by live canvas cursor requests before retained desktop platform
 cursor bridges continue.
+`pp_platform_api::probe_asset_file_timestamp` now owns the retained asset mtime
+probe used by layout reload planning, with missing/existing file coverage.
 `pano_cli plan-clipboard-read` and `pano_cli plan-clipboard-write` expose the
 app-core clipboard text decisions used by live clipboard get/set requests
 before retained platform clipboard bridges continue.

src/platform_api/asset_file_load_policy.cpp

@@ -5,6 +5,19 @@
 
 namespace pp::platform {
 
+AssetFileTimestampProbe probe_asset_file_timestamp(std::string_view absolute_path)
+{
+    struct stat file_info {};
+    const std::string path(absolute_path);
+    if (stat(path.c_str(), &file_info) != 0)
+        return {};
+
+    return {
+        true,
+        static_cast<std::int64_t>(file_info.st_mtime),
+    };
+}
+
 AssetFileLoadDecision plan_asset_file_load_with_probe(
     PlatformFamily family,
     bool already_loaded,
@@ -43,25 +56,11 @@ AssetFileLoadDecision plan_asset_file_load_for_platform(
             {});
     }
 
-    struct stat file_info {};
-    const std::string path(absolute_path);
-    if (stat(path.c_str(), &file_info) != 0)
-    {
     return plan_asset_file_load_with_probe(
         family,
         already_loaded,
         previous_last_write_time,
-            {});
-    }
-
-    return plan_asset_file_load_with_probe(
-        family,
-        already_loaded,
-        previous_last_write_time,
-        {
-            true,
-            static_cast<std::int64_t>(file_info.st_mtime),
-        });
+        probe_asset_file_timestamp(absolute_path));
 }
 
 AssetFileLoadDecision plan_asset_file_load(

src/platform_api/asset_file_load_policy.h

@@ -18,6 +18,9 @@ struct AssetFileTimestampProbe {
     std::int64_t last_write_time = 0;
 };
 
+[[nodiscard]] AssetFileTimestampProbe probe_asset_file_timestamp(
+    std::string_view absolute_path);
+
 [[nodiscard]] AssetFileLoadDecision plan_asset_file_load_with_probe(
     PlatformFamily family,
     bool already_loaded,

tests/platform_api/platform_services_tests.cpp

@@ -799,6 +799,27 @@ void asset_file_load_policy_preserves_platform_reload_behavior(pp::tests::Harnes
     std::filesystem::remove(temp_path);
 }
 
+void asset_file_load_policy_exposes_timestamp_probe(pp::tests::Harness& harness)
+{
+    const auto temp_path = std::filesystem::temp_directory_path()
+        / "panopainter-platform-api-timestamp-probe.xml";
+    std::filesystem::remove(temp_path);
+
+    const auto missing = pp::platform::probe_asset_file_timestamp(temp_path.string());
+    PP_EXPECT(harness, !missing.file_exists);
+
+    {
+        std::ofstream file(temp_path);
+        file << "<layouts />";
+    }
+
+    const auto existing = pp::platform::probe_asset_file_timestamp(temp_path.string());
+    PP_EXPECT(harness, existing.file_exists);
+    PP_EXPECT(harness, existing.last_write_time > 0);
+
+    std::filesystem::remove(temp_path);
+}
+
 void asset_file_load_policy_preserves_family_reload_behavior(pp::tests::Harness& harness)
 {
     const auto missing_desktop = pp::platform::plan_asset_file_load_with_probe(
@@ -1074,6 +1095,9 @@ int main()
     harness.run(
         "asset file load policy preserves platform reload behavior",
         asset_file_load_policy_preserves_platform_reload_behavior);
+    harness.run(
+        "asset file load policy exposes timestamp probe",
+        asset_file_load_policy_exposes_timestamp_probe);
     harness.run(
         "asset file load policy preserves family reload behavior",
         asset_file_load_policy_preserves_family_reload_behavior);