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_success.c

@@ -0,0 +1,44 @@
+#include "akerror.h"
+#include "err_capture.h"
+
+/*
+ * The success path: a nested call that returns cleanly (NULL) must not break
+ * out of the caller's ATTEMPT block, must not enter any handler, and must not
+ * consume a slot from the error pool.
+ */
+
+static int default_fired = 0;
+
+akerr_ErrorContext *ok_func(void)
+{
+    PREPARE_ERROR(e);
+    ATTEMPT {
+    } CLEANUP {
+    } PROCESS(e) {
+    } FINISH(e, true);
+    SUCCEED_RETURN(e);
+}
+
+int main(void)
+{
+    akerr_capture_install();
+    akerr_init();
+
+    int reached_after_catch = 0;
+    PREPARE_ERROR(e);
+    ATTEMPT {
+	CATCH(e, ok_func());
+	reached_after_catch = 1; /* must run: no break on success */
+    } CLEANUP {
+    } PROCESS(e) {
+    } HANDLE_DEFAULT(e) {
+	default_fired = 1; /* must NOT run */
+    } FINISH_NORETURN(e);
+
+    AKERR_CHECK(reached_after_catch == 1);
+    AKERR_CHECK(default_fired == 0);
+    AKERR_CHECK(e == NULL);
+    AKERR_CHECK(akerr_slots_in_use() == 0);
+    fprintf(stderr, "err_success ok\n");
+    return 0;
+}