Rename Step Line to Gig Line

Update the game name in project.godot, export presets, UI/marquee/title strings, packaging (AppImage/NSIS/desktop/appdata), tools, docs, and the generated prop GLBs. Packaging assets and README screenshots are regenerated. Headless suite 209/209 and Python tooling tests pass.

export_presets.cfg

@@ -9,7 +9,7 @@ custom_features=""
 export_filter="all_resources"
 include_filter=""
 exclude_filter=""
-export_path="build/step-line-linux.x86_64"
+export_path="build/gig-line-linux.x86_64"
 patches=PackedStringArray()
 encryption_include_filters=""
 encryption_exclude_filters=""
@@ -40,7 +40,7 @@ custom_features=""
 export_filter="all_resources"
 include_filter=""
 exclude_filter=""
-export_path="build/step-line-windows.exe"
+export_path="build/gig-line-windows.exe"
 patches=PackedStringArray()
 encryption_include_filters=""
 encryption_exclude_filters=""
@@ -76,7 +76,7 @@ custom_features=""
 export_filter="all_resources"
 include_filter=""
 exclude_filter=""
-export_path="build/step-line-macos.zip"
+export_path="build/gig-line-macos.zip"
 patches=PackedStringArray()
 encryption_include_filters=""
 encryption_exclude_filters=""
@@ -96,7 +96,7 @@ texture_format/etc2_astc=true
 codesign/enable=false
 codesign/timestamp=true
 codesign/identity_type=0
-application/bundle_identifier="biz.allucanget.step-line"
+application/bundle_identifier="biz.allucanget.gig-line"
 application/icon=""
 application/icon_interpolation=4
 application/export_angle=0