fix: Default to day-first format for invalid/unknown locales (#1168)

## Summary
Fixes a bug where invalid/unknown locales would default to month-first
(American) format instead of day-first format. Since most of the world
uses day-first, this is the better default.

## Problem
When an invalid or unknown locale was provided (e.g., typo, unsupported
locale), the exception handler would return `False` (month-first):

```python
except Exception:
    return False  # ❌ Defaults to month-first (MM/DD/YYYY)
```

This caused incorrect parsing:
- User provides `locale="xyz_ABC"` (typo)
- Date `"01/02/2023"` gets parsed as **January 2nd** (month-first)
- Should be parsed as **February 1st** (day-first, more common globally)

### Why this is wrong
Only a handful of locales use month-first format:
- `en_US` - American English
- A few Pacific island locales

**Most of the world uses day-first:**
- All of Europe
- Most of Asia
- Middle East
- Africa
- South America
- Australia

## Solution
Changed exception handler to return `True` (day-first) for invalid
locales:

```python
except Exception:
    # Invalid/unknown locale: default to day-first since most of the world uses it
    # Only en_US and a few other locales use month-first (MM/DD/YYYY)
    return True  # ✅ Defaults to day-first (DD/MM/YYYY)
```

### The logic now:
1. **ISO 8601** (`YYYY-MM-DD`) → `False` (month before day)
2. **No locale** (`None`) → `False` (backward compat, American default)
3. **Valid locale** → Check Babel CLDR data
4. **Invalid locale** → `True` (day-first is global default)

## Code Changes

**Before:**
```python
except Exception:
    return False  # Defaulted to month-first
```

**After:**
```python
except Exception:
    # Invalid/unknown locale: default to day-first since most of the world uses it
    # Only en_US and a few other locales use month-first (MM/DD/YYYY)
    return True  # Default to day-first (DD/MM/YYYY)
```

## Testing

**Updated test:**
```python
def test_invalid_locale_returns_true(self) -> None:
    # Invalid/unknown locales default to day-first since most of world uses it
    assert _should_use_day_first("01/15/2023", "invalid_LOCALE") is True
    assert _should_use_day_first("15/01/2023", "xyz_ABC") is True
```

**New integration test:**
```python
def test_invalid_locale_defaults_to_day_first(self) -> None:
    parser = TimestampParser()
    
    # With an invalid locale, ambiguous dates should parse as day-first
    with set_locale_context("invalid_LOCALE"):
        result = parser.parse("01/02/2023")
    assert "2023-02-01" in result  # February 1st (day-first)
```

- ✅ All 19 timestamp parser tests pass
- ✅ All 547 tests pass
- ✅ Linting clean

## Impact
- **Fixes bug** - Invalid locales now use the globally more common
format
- **Better user experience** - Fewer parsing errors for international
users
- **No breaking changes** - Only affects invalid locale case (which was
already broken)
- **Backward compatible** - `None` locale still defaults to American
format

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.1 <noreply@anthropic.com>

Author: nathan-stender

src/allotropy/parsers/utils/timestamp_parser.py

@@ -42,7 +42,7 @@ def _should_use_day_first(time_str: str, locale_str: str | None) -> bool:
     if re.match(r"^\d{4}[-/.]", time_str):
         return False
 
-    # Use locale data for non-ISO formats
+    # No locale specified: default to American format for backward compatibility
     if not locale_str:
         return False
 
@@ -60,7 +60,9 @@ def _should_use_day_first(time_str: str, locale_str: str | None) -> bool:
 
         return False
     except Exception:
-        return False
+        # Invalid/unknown locale: default to day-first since most of the world uses it
+        # Only en_US and a few other locales use month-first (MM/DD/YYYY)
+        return True
 
 
 class TimestampParser:

tests/parsers/utils/timestamp_parser_test.py

@@ -42,9 +42,11 @@ def test_none_locale_returns_false(self) -> None:
         assert _should_use_day_first("01/15/2023", None) is False
         assert _should_use_day_first("15/01/2023", None) is False
 
-    def test_invalid_locale_returns_false(self) -> None:
-        # Invalid locales default to False
-        assert _should_use_day_first("01/15/2023", "invalid_LOCALE") is False
+    def test_invalid_locale_returns_true(self) -> None:
+        # Invalid/unknown locales default to day-first since most of world uses it
+        assert _should_use_day_first("01/15/2023", "invalid_LOCALE") is True
+        assert _should_use_day_first("15/01/2023", "xyz_ABC") is True
+        assert _should_use_day_first("01/02/2023", "not_a_real_locale") is True
 
 
 class TestTimestampParser:
@@ -142,6 +144,21 @@ def test_parse_invalid_date_raises_error(self) -> None:
         ):
             parser.parse("not a date")
 
+    def test_invalid_locale_defaults_to_day_first(self) -> None:
+        """Invalid/unknown locales should default to day-first (most common globally)."""
+        parser = TimestampParser()
+
+        # With an invalid locale, ambiguous dates should parse as day-first
+        # 01/02/2023 should be February 1st (day-first), not January 2nd
+        with set_locale_context("invalid_LOCALE"):
+            result = parser.parse("01/02/2023")
+        assert "2023-02-01" in result, "Invalid locale should default to day-first"
+
+        # Another invalid locale - should still use day-first
+        with set_locale_context("xyz_ABC"):
+            result = parser.parse("15/01/2023")
+        assert "2023-01-15" in result, "Unknown locale should default to day-first"
+
     def test_real_world_examples(self) -> None:
         """Test real-world date formats from various instruments."""
         parser = TimestampParser()