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/0012-basic-shadow.sx

@@ -0,0 +1,21 @@
+#import "modules/std.sx";
+main :: () -> i32 {
+    x := 42;
+    {
+        print("scope opened\n");
+        defer print("scope closed\n");
+
+        // define a inner variable x shadowing the one define in the outer scope(s)
+        x:= 6;
+        print("scoped x: {}\n", x); //expect 6
+    }
+    print("main x: {}\n", x); //expect 42
+    0
+}
+
+// ** stdout **
+// scope opened
+// scoped x: 6
+// scope closed
+// main x: 42
+//