rugnux --model: say why validation did not run when the data carry no resolution

The d_min <= 0 return was the one failure path that left failure_reason empty.
The report's MODEL_VALIDATION= NOT_PERFORMED line and the WARNING that goes with
it are both gated on that string, so this case printed a blank reason and no
warning - the "indistinguishable from a run that was never given --model" outcome
the field was added to prevent. All four failure returns now set it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016L1qig74oYQzfUJJZbbxFh

rugnux/ModelValidation.cpp

@@ -137,7 +137,8 @@ ModelValidationResult ValidateAgainstModel(const std::vector<MergedReflection> &
         if (r.d > 0 && (d_min == 0.0 || r.d < d_min))
             d_min = r.d;
     if (d_min <= 0.0) {
-        logger.Error("Model validation: merged reflections carry no resolution");
+        result.failure_reason = "the merged reflections carry no resolution";
+        logger.Error("Model validation: {}", result.failure_reason);
         return result;
     }