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/diagnostics/1157-diagnostics-catch-binding-needs-parens.sx

@@ -0,0 +1,14 @@
+// The catch error binding is parenthesized — `catch (e) { }`; a bare
+// binding is rejected with a migration hint. (Same rule as the for-loop
+// capture; `onfail (e) { }` follows it too.)
+
+#import "modules/std.sx";
+
+E :: error { Bad };
+
+f :: () -> (i64, !E) { raise error.Bad; }
+
+main :: () {
+    v := f() catch e { 0 };
+    print("{}\n", v);
+}