feat(editor): in-editor "Snap All Spawns to Ground" menu

Adds EditorApp::snapAllSpawnsToGround() and a menu item under
Generate, mirroring the --snap-zone-to-ground CLI for users who
want the action without context-switching to a terminal.

Walks every NPC + object, casts a downward ray from baseZ+500y,
and writes the hit Z into spawn.position.z. Patrol waypoints on
each NPC are snapped too. Marks the project dirty + auto-save
pending so the change persists without a manual Ctrl+S.

Existing per-selection "Snap Ground" button on the right panel is
unchanged; this is the bulk version for "I just edited terrain
under a populated area."

tools/editor/editor_app.hpp

@@ -102,6 +102,10 @@ public:
     // tile's world bbox. Mirrors the --random-populate-zone CLI
     // command so users can do bulk population from inside the editor.
     void randomPopulateZone(int creatureCount, int objectCount, uint32_t seed);
+    // Re-snap every creature + object Z to actual terrain. In-editor
+    // mirror of the --snap-zone-to-ground CLI; useful after terrain
+    // edits or random population to fix floating/buried spawns.
+    void snapAllSpawnsToGround();
     void centerOnTerrain();
 
     // Multi-tile support