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_pool_refcount.c

@@ -0,0 +1,55 @@
+#include "akerror.h"
+#include "err_capture.h"
+
+/*
+ * Pool-hygiene regression test. The error pool is a fixed 128-slot static
+ * array; a single leaked reference per error would silently exhaust it. Run a
+ * large number of full raise -> catch -> handle cycles and confirm every slot
+ * is returned to the pool (refcount 0) and the pool can still hand out
+ * contexts afterwards.
+ */
+
+#define ITERATIONS 100000
+
+akerr_ErrorContext *boom(void)
+{
+    PREPARE_ERROR(e);
+    ATTEMPT {
+	FAIL(e, AKERR_VALUE, "boom %d", 1);
+    } CLEANUP {
+    } PROCESS(e) {
+    } FINISH(e, true);
+    SUCCEED_RETURN(e);
+}
+
+/* One full raise -> catch -> handle cycle. Returns NULL (context released). */
+akerr_ErrorContext *one_cycle(void)
+{
+    PREPARE_ERROR(e);
+    ATTEMPT {
+	CATCH(e, boom());
+    } CLEANUP {
+    } PROCESS(e) {
+    } HANDLE(e, AKERR_VALUE) {
+    } FINISH(e, false);
+    return e;
+}
+
+int main(void)
+{
+    akerr_capture_install();
+    akerr_init();
+    AKERR_CHECK(akerr_slots_in_use() == 0);
+
+    for ( int iter = 0; iter < ITERATIONS; iter++ ) {
+	(void)one_cycle();
+    }
+
+    AKERR_CHECK(akerr_slots_in_use() == 0);
+
+    akerr_ErrorContext *probe = akerr_next_error();
+    AKERR_CHECK(probe != NULL);
+
+    fprintf(stderr, "err_pool_refcount ok (%d cycles, 0 leaked)\n", ITERATIONS);
+    return 0;
+}