Revert generic grid_overlay protection from config_file::save()

Per Nir-Az's review comment: the grid/crosshair overlay is a
viewer_model-scoped feature and has no business special-casing the
generic config_file save/set path used by every other feature (device
options, DDS settings, window geometry, ...). Removes
_grid_overlay_dirty, is_grid_overlay_path(), and the disk-read/merge
that save() ran on every flush to protect that one section.

This intentionally re-opens the narrower issue the removed code was
guarding against: hand-editing viewer_model.grid_overlay.* in
realsense-config.json while the viewer is running can still be
silently clobbered by the next unrelated config save. Left unaddressed
here to keep this PR scoped to the crosshair feature; worth a separate,
narrowly-targeted fix if it's worth solving at all.

Author: Evgeni Raikhel

common/rs-config.cpp

@@ -113,7 +113,6 @@ config_file& config_file::instance()
 config_file::config_file( std::string const & filename )
     : _filename( filename )
     , _dirty( false )
-    , _grid_overlay_dirty( false )
     , _save_stop( false )
 {
     try
@@ -128,39 +127,9 @@ config_file::config_file( std::string const & filename )
     _save_thread = std::thread( &config_file::save_loop, this );
 }
 
-bool config_file::is_grid_overlay_path( const std::string & path )
-{
-    static const std::string prefix = "viewer_model.grid_overlay.";
-    return path.rfind( prefix, 0 ) == 0;
-}
-
 void config_file::save()
 {
     std::lock_guard< std::recursive_mutex > lk( _mutex );
-
-    // If nothing in this process has intentionally changed the crosshair/grid overlay
-    // section since the last flush, adopt whatever is currently on disk for it before
-    // writing. This flush can be triggered by any unrelated set() (window move/resize,
-    // device options, ...) up to once per SAVE_INTERVAL; without this, it would blindly
-    // overwrite a hand-edit made to realsense-config.json while the viewer is running
-    // with our stale in-memory copy.
-    if( ! _grid_overlay_dirty.exchange( false ) && ! _filename.empty() )
-    {
-        try
-        {
-            auto on_disk = rsutils::json_config::load_from_file( _filename );
-            if( on_disk.exists() && on_disk.contains( "viewer_model" )
-                && on_disk["viewer_model"].contains( "grid_overlay" ) )
-            {
-                _j["viewer_model"]["grid_overlay"] = on_disk["viewer_model"]["grid_overlay"];
-            }
-        }
-        catch( ... )
-        {
-            // Best effort - if the file can't be read, fall back to whatever _j already has.
-        }
-    }
-
     if( ! _filename.empty() )
         save( _filename.c_str() );
 }
@@ -194,7 +163,6 @@ void config_file::save_loop()
 config_file::config_file()
     : _j( rsutils::json::object() )
     , _dirty( false )
-    , _grid_overlay_dirty( false )
     , _save_stop( false )
 {
 }
@@ -214,12 +182,7 @@ config_file& config_file::operator=(const config_file& other)
         std::lock_guard< std::recursive_mutex > lk_this( _mutex );
         _j = std::move( j_copy );
         _defaults = std::move( defaults_copy );
-        // Assignment is always an intentional, wholesale replacement (Load Settings,
-        // Restore Defaults + Apply, etc.) - mark grid_overlay dirty too, or the next
-        // deferred flush would discard whatever value came with this assignment by
-        // merging stale on-disk content back over it.
         _dirty = true;
-        _grid_overlay_dirty = true;
     }
     return *this;
 }

common/rs-config.h

@@ -156,8 +156,6 @@ namespace rs2
 
             ( *current )[keys.back()] = val;
             _dirty = true;
-            if( is_grid_overlay_path( path ) )
-                _grid_overlay_dirty = true;
         }
 
         // Sets a default value to the config and default map
@@ -201,8 +199,6 @@ namespace rs2
                 }
                 ( *current )[keys.back()] = default_val;
                 _dirty = true;
-                if( is_grid_overlay_path( path ) )
-                    _grid_overlay_dirty = true;
             }
         }
 
@@ -214,11 +210,6 @@ namespace rs2
 
         static constexpr std::chrono::milliseconds SAVE_INTERVAL{ 1000 };
 
-        // True if `path` (dot-notation) falls under the viewport grid/crosshair overlay
-        // section - the one config section save() protects from being overwritten by an
-        // unrelated flush; see _grid_overlay_dirty and save().
-        static bool is_grid_overlay_path( const std::string & path );
-
         // Serializes all reads/writes of `_j` and the on-disk file. Required because
         // viewer reads/writes config_file from multiple threads (UI thread, the
         // config_save_worker background thread in subdevice-model.cpp, and ad-hoc
@@ -230,7 +221,6 @@ namespace rs2
         std::string _filename;
         rsutils::json _j;
         std::atomic<bool> _dirty;
-        std::atomic<bool> _grid_overlay_dirty;
         std::condition_variable _save_cv;
         std::mutex _save_cv_mutex;
         bool _save_stop;