test: group examples into per-category folders

Move examples/*.sx and their expected/ snapshots into per-category
subfolders (examples/<category>/...). Folder = leading filename token,
with ffi-objc/ffi-jni kept whole; filenames are unchanged. The corpus
runner and LSP sweep now discover each category's expected/ dir, while
issues/ stays flat. Example 1058's repo-root-relative companion import
is made file-relative. Path strings embedded in 164 snapshots were
regenerated (path-only changes). Test-layout docs in CLAUDE.md updated.

examples/platform/1600-platform-graphics.sx

@@ -0,0 +1,19 @@
+#import "modules/ffi/raylib.sx";
+
+main :: () {
+    InitWindow(800, 600, "sx - Triangle");
+
+    while !WindowShouldClose() {
+        BeginDrawing();
+        ClearBackground(Color.{50, 50, 50, 255});
+
+        v1 := Vector2.{400.0, 100.0};
+        v2 := Vector2.{200.0, 500.0};
+        v3 := Vector2.{600.0, 500.0};
+        DrawTriangle(v1, v2, v3, Color.{230, 41, 55, 255});
+
+        EndDrawing();
+    }
+
+    CloseWindow();
+}