Document scene checksum repairs before playtests

docs/ARCHITECTURE.md

@@ -534,8 +534,12 @@ write-file, flush, atomic rename, and recoverable backup. Unknown future version
 read-only. When opening a scene for authoring, a stale checksum on otherwise valid
 data produces a Console warning and keeps the primary file editable. This supports
 external text edits without silently loading an older backup. Saving refreshes the
-checksum and retains the previous file as `.bak`. Invalid components/environment
-settings still trigger recovery or read-only access; strict non-authoring loaders
+checksum and retains the previous file as `.bak`. The editor tracks a checksum-only
+mismatch as pending persistence so autosave can repair it without a local edit.
+Playtest startup saves a pending checksum repair in writable projects, even with
+autosave disabled. Successful saves refresh load diagnostics; failed saves retain
+the pending repair. Read-only projects never rewrite the source scene. Invalid
+components/environment settings still trigger recovery or read-only access; strict non-authoring loaders
 continue to reject checksum mismatches. Entity/component ordering is deterministic
 for useful Git/Perforce diffs.
 Large source binaries use Git LFS or Perforce; `cache`, `temp`, `logs`, `.rustic/local`,

docs/SCENE_ENVIRONMENT/set-up-a-sky.md

@@ -4,7 +4,7 @@
 
 To choose the scene to edit, use the project settings gear → **Change scene**. Right-click a scene and choose **Set as start scene** to open it automatically the next time you open the project. Without a saved choice, the editor opens `main.scene`, then legacy `main.rscene`, then the top scene in the sorted list, including subfolders. Projects with no scene files open an editable empty Main scene; saving creates `main.scene`. If a saved start scene is missing, restore that file or remove the optional `start_scene` field in `project.engine` to restore automatic selection.
 
-If the project opens read-only because another editor has it open, close that editor and reopen the project. A crashed editor releases its writer lock automatically; leftover lock metadata does not prevent editing. Switching scenes retains the project writer lock. If a scene becomes read-only, check Console entries under `scene.load`: they now show the file path and the validation reason. A valid external text edit may leave a stale checksum; the editor warns about that mismatch but keeps the edited primary scene writable. Save to refresh its checksum; the prior file is retained as `.bak`. Invalid components/environment settings and newer unsupported formats still require recovery or read-only access.
+If the project opens read-only because another editor has it open, close that editor and reopen the project. A crashed editor releases its writer lock automatically; leftover lock metadata does not prevent editing. Switching scenes retains the project writer lock. If a scene becomes read-only, check Console entries under `scene.load`: they now show the file path and the validation reason. A valid external text edit may leave a stale checksum; the editor warns about that mismatch but keeps the edited primary scene writable. Save to refresh its checksum; the prior file is retained as `.bak`. A valid scene with a stale checksum is now marked as needing a save even when you have made no local edits. Autosave repairs it when enabled; starting Play, New Window, or Standalone also saves the pending repair before launching, including when autosave is off. A successful save clears the resolved load diagnostic and keeps the externally edited file as `.bak`. If saving fails, the editor reports the write failure and keeps the repair pending; fix the destination or permissions and try again. Read-only projects do not rewrite the source scene; their play snapshots still receive a fresh checksum. Invalid components/environment settings and unsupported future formats are not repaired this way. Invalid components/environment settings and newer unsupported formats still require recovery or read-only access.
 
 1. Copy a **2:1 equirectangular panorama** into your project, for example `assets/sky/sunset.hdr`. Supported formats are Radiance HDR (`.hdr`), PNG, JPEG and TGA. A 4096 × 2048 image is a typical choice; each dimension must be at most 8192 pixels. Six separate cubemap faces, EXR and model files are not supported.
 2. Open the Inspector and expand **Scene sky & atmosphere**, above the selected entity's properties. You can also use it with nothing selected.