Expand test coverage for error-handling macros and error pool

Add 11 CTest programs and a shared test helper covering gaps left by the
original four tests (which only exercised FAIL/CATCH/HANDLE and CLEANUP):

- err_capture.h: capturing akerr_log_method + NDEBUG-proof AKERR_CHECK so
  tests can assert on message/status/stacktrace content, not just exit codes
- err_success: clean nested return does not break/handle/leak
- err_pool_refcount: 100k raise->catch->handle cycles leak 0 pool slots
- err_handle_default / err_handle_group / err_handle_dispatch: handler routing
- err_pass / err_ignore / err_swallow: PASS, IGNORE, FINISH(e,false)
- err_break_variants: FAIL_*_BREAK and FAIL_*_RETURN
- err_errno: system errno name lookup + "Unknown Error" boundary
- err_custom_handler: override the unhandled-error hook, assert non-fatally

Register tests via a foreach loop in CMakeLists.txt. Ignore build/.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/err_custom_handler.c

@@ -0,0 +1,45 @@
+#include "akerror.h"
+#include "err_capture.h"
+
+/*
+ * The unhandled-error hook (akerr_handler_unhandled_error) is overridable.
+ * Install a non-fatal handler so an unhandled error can be asserted directly
+ * (status, invocation) instead of relying on process death / WILL_FAIL.
+ */
+
+static int custom_fired = 0;
+static int custom_status = 0;
+
+static void my_handler(akerr_ErrorContext *e)
+{
+    custom_fired = 1;
+    custom_status = (e != NULL) ? e->status : -1;
+    /* deliberately does NOT exit() */
+}
+
+akerr_ErrorContext *boom(void)
+{
+    PREPARE_ERROR(e);
+    FAIL_RETURN(e, AKERR_TYPE, "unhandled on purpose");
+}
+
+int main(void)
+{
+    akerr_capture_install();
+    akerr_init(); /* sets the default handler... */
+    akerr_handler_unhandled_error = &my_handler; /* ...which we then override */
+
+    PREPARE_ERROR(e);
+    ATTEMPT {
+	CATCH(e, boom());
+    } CLEANUP {
+    } PROCESS(e) {
+	/* no HANDLE for AKERR_TYPE -> stays unhandled */
+    } FINISH_NORETURN(e);
+
+    AKERR_CHECK(custom_fired == 1);
+    AKERR_CHECK(custom_status == AKERR_TYPE);
+    AKERR_CHECK_CONTAINS("Unhandled Error");
+    fprintf(stderr, "err_custom_handler ok\n");
+    return 0;
+}