test: run example corpus in zig build test; sx ir → stdout

`zig build test` now runs the full examples/ + issues/ regression corpus
alongside the Zig unit tests, driven by a pure-Zig test
(src/corpus_run.test.zig) — no shell script in the build path. It spawns
the installed `sx` per example (subprocess-isolated, per-run timeout),
diffs stdout/stderr/exit and optional `sx ir` snapshots, and fails the
build on any mismatch. The file list is enumerated at runtime, so new
examples are covered with no test edit.

- `sx ir` / `ir-dump` now write to stdout (fd 1) instead of stderr, so
  the dumps can be piped/redirected.
- `zig build test -Dupdate-goldens` regenerates snapshots in-build,
  byte-identical to the legacy `run_examples.sh --update`; on mismatch
  the runner prints how to regenerate.
- run_examples.sh kept (still used by tools/verify-step.sh) and made
  portable to a bare macOS: timeout/gtimeout fallback, bash 3.2-safe
  empty-array handling.
- CLAUDE.md: document the new workflow.

src/main.zig

@@ -520,7 +520,9 @@ fn dumpSxIR(allocator: std.mem.Allocator, io: std.Io, input_path: []const u8, st
     sx.ir.printModule(&ir_module, &aw.writer) catch return error.CompileError;
     var result = aw.writer.toArrayList();
     defer result.deinit(allocator);
-    std.debug.print("{s}", .{result.items});
+    // Emit to stdout (fd 1), not stderr: `ir-dump` is a data-emitting command
+    // meant to be piped/redirected. Matches `sx ir`'s stdout routing.
+    _ = std.c.write(1, result.items.ptr, result.items.len);
 }
 
 fn emitIR(allocator: std.mem.Allocator, io: std.Io, input_path: []const u8, target_config: sx.target.TargetConfig, stdlib_paths: []const []const u8) !void {