# Use nil-sentinel dataclasses instead of None (zero-initialized frozen dataclass; safe to read fields)

### 1. Nil-Sentinel Dataclasses (replaces `None`)

When a function would "return None" in conventional Python, return a
nil-sentinel dataclass instead. The sentinel has all default values
(zero-initialized) and is safe to read from.

```python
from dataclasses import dataclass, field

@dataclass(frozen=True)
class NilPath:
 exists: bool = False
 read_text: str = ""
 errors: list[ErrorInfo] = field(default_factory=list)

NIL_PATH = NilPath() # module-level singleton
```

Callers don't need `if x is None:` checks; they can call `x.read_text` and
get `""` on the nil path.

**Convention:** `NIL_*` (uppercase) is the module-level singleton. `Nil*`
(PascalCase) is the class. Frozen dataclass prevents runtime mutation.