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/errors/1010-errors-catch-rejections.sx

@@ -0,0 +1,13 @@
+// `catch` rejection (ERR step E1.5): the operand must be failable. Unlike
+// `try` / `raise`, `catch` needs no failable enclosing function — it consumes
+// the error locally — so the only stable rejection is a non-failable operand.
+// The positive cases live in `examples/226-catch.sx`.
+
+#import "modules/std.sx";
+
+plain :: () -> i32 { return 0; }
+
+main :: () -> i32 {
+    plain() catch (e) { return 1; };   // error: operand has type i32 (not failable)
+    return 0;
+}