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_ignore.c

@@ -0,0 +1,34 @@
+#include "akerror.h"
+#include "err_capture.h"
+
+/*
+ * IGNORE deliberately swallows an error: it records the context in
+ * __akerr_last_ignored, logs it with an "IGNORED ERROR" marker, and lets
+ * execution continue.
+ */
+
+akerr_ErrorContext *boom(void)
+{
+    PREPARE_ERROR(e);
+    FAIL_RETURN(e, AKERR_VALUE, "this error is ignored on purpose");
+}
+
+int main(void)
+{
+    akerr_capture_install();
+
+    int reached_after_ignore = 0;
+    PREPARE_ERROR(e);
+    (void)e;
+
+    IGNORE(boom());
+    reached_after_ignore = 1;
+
+    AKERR_CHECK(__akerr_last_ignored != NULL);
+    AKERR_CHECK(__akerr_last_ignored->status == AKERR_VALUE);
+    AKERR_CHECK(reached_after_ignore == 1);
+    AKERR_CHECK_CONTAINS("IGNORED ERROR");
+    AKERR_CHECK_CONTAINS("this error is ignored on purpose");
+    fprintf(stderr, "err_ignore ok\n");
+    return 0;
+}