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/1031-errors-process-exit.sx

@@ -0,0 +1,15 @@
+// `process.exit` (ERR step E4.1): immediate process termination with an exit
+// code. No `defer` / `onfail` cleanup runs and no error-trace frames are pushed
+// — it's POSIX `_exit(2)`. Here a runtime call exits 42; the line after never
+// runs. (sx `print` writes unbuffered via `write(2)`, so the "starting" line
+// still appears despite `_exit` skipping the stdio flush.) Expected exit: 42.
+
+#import "modules/std.sx";
+proc :: #import "modules/std/process.sx";
+
+main :: () -> i32 {
+    print("starting\n");
+    proc.exit(42);
+    print("unreachable\n");
+    return 0;
+}