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/memory/0803-memory-push-context-with-arena.sx

@@ -0,0 +1,17 @@
+// `push <Context>` where Context's first field is an `#inline` protocol
+// (`allocator: Allocator`) and the value being pushed is an `Arena` upcast to
+// that protocol. Exercises save/restore of the boxed context across the push.
+
+#import "modules/std.sx";
+#import "modules/std/mem.sx";
+
+main :: () -> void {
+    arena := Arena.init(context.allocator, 4096);
+
+    new_ctx := Context.{ allocator = xx arena, data = context.data };
+    push new_ctx {
+        ptr := context.allocator.alloc_bytes(128);
+        out("inside push\n");
+    }
+    out("after push\n");
+}