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_handle_group.c

@@ -0,0 +1,56 @@
+#include "akerror.h"
+#include "err_capture.h"
+
+/*
+ * HANDLE_GROUP lets several status codes share one handler body via
+ * case-fallthrough. The first member of the group uses HANDLE (which emits the
+ * leading break that terminates the previous case); each additional member
+ * uses HANDLE_GROUP; the shared body follows the last member. Verify that two
+ * different status codes both reach the shared body.
+ */
+
+static int group_fired = 0;
+static int other_fired = 0;
+
+akerr_ErrorContext *boom(int status)
+{
+    PREPARE_ERROR(e);
+    FAIL_RETURN(e, status, "boom %d", status);
+}
+
+akerr_ErrorContext *run(int status)
+{
+    PREPARE_ERROR(e);
+    ATTEMPT {
+	CATCH(e, boom(status));
+    } CLEANUP {
+    } PROCESS(e) {
+    } HANDLE(e, AKERR_KEY)
+      HANDLE_GROUP(e, AKERR_INDEX) {
+	group_fired++;
+    } HANDLE(e, AKERR_IO) {
+	other_fired++;
+    } FINISH(e, false);
+    return e;
+}
+
+int main(void)
+{
+    akerr_capture_install();
+
+    run(AKERR_KEY);
+    AKERR_CHECK(group_fired == 1);
+    AKERR_CHECK(other_fired == 0);
+
+    run(AKERR_INDEX);
+    AKERR_CHECK(group_fired == 2);
+    AKERR_CHECK(other_fired == 0);
+
+    run(AKERR_IO);
+    AKERR_CHECK(group_fired == 2);
+    AKERR_CHECK(other_fired == 1);
+
+    AKERR_CHECK(akerr_slots_in_use() == 0);
+    fprintf(stderr, "err_handle_group ok\n");
+    return 0;
+}