Fix format-string use of __FILE__/__func__ and name_for_status lower bound

Two hardening fixes flagged by the earlier review:

1. FAIL passed __FILE__ and __func__ directly as the snprintf format string.
   __FILE__ expands to a string literal that could contain a '%' (a build path
   under a directory with a percent sign), and __func__ is not a literal at all;
   either way snprintf would read nonexistent varargs. Pass them as "%s"
   arguments instead.

2. akerr_name_for_status guarded the upper bound but not the lower one, so a
   negative status indexed __AKERR_ERROR_NAMES[negative] -- an out-of-bounds
   read, or an out-of-bounds write when a name was supplied. Reject status < 0.

Regression tests err_format_string (uses #line to put a conversion specifier in
__FILE__) and err_name_bounds fail against the old code (verified) and pass now.
Full suite: 23/23, no warnings.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/err_format_string.c

@@ -0,0 +1,34 @@
+#include "akerror.h"
+#include "err_capture.h"
+#include <string.h>
+
+/*
+ * FAIL records the source file and function names with snprintf. Those names
+ * must be passed as %s ARGUMENTS, not used as the format string -- otherwise a
+ * path containing a printf conversion (say a build directory with a '%') is
+ * interpreted as a format and reads nonexistent varargs (undefined behavior).
+ *
+ * #line lets us make __FILE__ contain a conversion specifier; the stored name
+ * must come back verbatim.
+ */
+
+#line 1 "pct%dname.c"
+akerr_ErrorContext *raise_with_percent_in_filename(void)
+{
+    PREPARE_ERROR(e);
+    FAIL_RETURN(e, AKERR_VALUE, "boom");
+}
+#line 22 "tests/err_format_string.c"
+
+int main(void)
+{
+    akerr_init();
+
+    akerr_ErrorContext *e = raise_with_percent_in_filename();
+    AKERR_CHECK(e != NULL);
+    AKERR_CHECK(strcmp(e->fname, "pct%dname.c") == 0);
+
+    e = akerr_release_error(e);
+    fprintf(stderr, "err_format_string ok\n");
+    return 0;
+}