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/basic/0013-basic-defer.sx

@@ -0,0 +1,11 @@
+#import "modules/std.sx";
+main :: () -> i32 {
+    defer print("still here\n");
+    return 42;
+}
+
+// ** exit code ** 
+// 42 
+// ** stdout **
+// still here
+//