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_swallow.c

@@ -0,0 +1,45 @@
+#include "akerror.h"
+#include "err_capture.h"
+
+/*
+ * FINISH(ctx, false) closes an ATTEMPT block without propagating: an unhandled
+ * error is dropped (not returned, not passed to the unhandled handler) and the
+ * context is released back to the pool. Because we use FINISH (not
+ * FINISH_NORETURN) the process must continue normally and exit 0.
+ */
+
+static int wrong_handler_fired = 0;
+
+akerr_ErrorContext *boom(void)
+{
+    PREPARE_ERROR(e);
+    FAIL_RETURN(e, AKERR_VALUE, "unhandled but swallowed");
+}
+
+/* FINISH(e, false) belongs in a context-returning function. On the swallow
+ * path e is released to NULL, so this returns NULL to its caller. */
+akerr_ErrorContext *swallow_it(void)
+{
+    PREPARE_ERROR(e);
+    ATTEMPT {
+	CATCH(e, boom());
+    } CLEANUP {
+    } PROCESS(e) {
+    } HANDLE(e, AKERR_KEY) {
+	wrong_handler_fired = 1; /* does not match AKERR_VALUE */
+    } FINISH(e, false);
+    return e; /* NULL: released even though unhandled */
+}
+
+int main(void)
+{
+    akerr_capture_install();
+
+    akerr_ErrorContext *res = swallow_it();
+
+    AKERR_CHECK(wrong_handler_fired == 0);
+    AKERR_CHECK(res == NULL); /* released even though unhandled */
+    AKERR_CHECK(akerr_slots_in_use() == 0);
+    fprintf(stderr, "err_swallow ok\n");
+    return 0;
+}