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/1603-platform-stb-image.sx

@@ -0,0 +1,16 @@
+#import "modules/std.sx";
+stb :: #import "vendors/stb_image/stb_image.sx";
+
+main :: () -> i32 {
+    w: i32 = 0;
+    h: i32 = 0;
+    ch: i32 = 0;
+    img := stb.stbi_load("test.png", @w, @h, @ch, 4);
+    if xx img != 0 {
+        print("loaded {}x{} ({} channels)\n", w, h, ch);
+        stb.stbi_image_free(xx img);
+    } else {
+        print("no image (expected in test)\n");
+    }
+    0
+}