Disable retrodiff when it was not explicitly enabled, and the pool doesn't seem to like it

If retrodiff is merely enabled by default, and the pool rejects such a share specifically because its hash failed to meet the target, it suggests it tracks job difficulties correctly, and we should stop with the retrodiff.
This has no effect if the user explicitly enables (or disables) retrodiff.

miner.c

@@ -1227,6 +1227,7 @@ bool pool_diff_effective_retroactively(struct pool * const pool)
 	}
 	
 	// By default, we enable retrodiff for stratum pools since some servers implement mining.set_difficulty in this way
+	// Note that share_result will explicitly disable BTS_UNKNOWN -> BTS_FALSE if a retrodiff share is rejected specifically for its failure to meet the target.
 	return pool->stratum_active;
 }
 
@@ -5260,6 +5261,15 @@ share_result(json_t *val, json_t *res, json_t *err, const struct work *work,
 		char reason[32];
 		put_in_parens(reason, sizeof(reason), extract_reject_reason(val, res, err, work));
 		applog(LOG_DEBUG, "Share above target rejected%s by pool %u as expected, ignoring", reason, pool->pool_no);
+		
+		// Stratum error 23 is "low difficulty share", which suggests this pool tracks job difficulty correctly.
+		// Therefore, we disable retrodiff if it was enabled-by-default.
+		if (pool->pool_diff_effective_retroactively == BTS_UNKNOWN) {
+			json_t *errnum;
+			if (work->stratum && err && json_is_array(err) && json_array_size(err) >= 1 && (errnum = json_array_get(err, 0)) && json_is_number(errnum) && ((int)json_number_value(errnum)) == 23) {
+				pool->pool_diff_effective_retroactively = false;
+			}
+		}
 	} else {
 		mutex_lock(&stats_lock);
 		cgpu->rejected++;