```python
from typing import List, Dict

def field_mean(records: List[Dict], field: str) -> int:
    """
    Calculate the mean of a specified numeric field across records, rounding down.
    
    Args:
    records: A list of dictionaries, where each dictionary represents a record.
    field: The name of the numeric field to calculate the mean for.
    
    Returns:
    The mean of the specified field across all records, rounded down to the nearest integer.
    If no records contain the field or if the field contains non-numeric values, returns 0.
    """
    total_sum = 0
    count = 0
    
    for record in records:
        if field in record and isinstance(record[field], (int, float)):
            total_sum += record[field]
            count += 1
    
    if count == 0:
        return 0
    
    return int(total_sum // count)
```