Copy-constructing a derived config from a base instance requires explicitly re-stating pinned field defaults

**Summary**

When a derived config re-declares a base field as `Pinned[T]`, copy-constructing the derived config from a base instance raises a `ValueError` unless the user explicitly passes `field=Pinned` (or `field=Pin(value)`) at the call site. The derived class's `pdefaults` entry alone is not sufficient to suppress the error.

**Motivation**

The derived class has already declared the field as pinned and set its default via `pdefaults`. Requiring the user to re-state the pinned default at every call site is redundant and easy to forget, producing a confusing error with no indication of what went wrong.

**Proposed behavior**

When copy-constructing a derived config from a base instance, fields declared `Pinned[T]` in the derived class should not require the user to re-state the pinned default at the call site. The derived class's `pdefaults` entry should be sufficient.

This applies when the pinned field is managed by a `@pproperty`:

```python
from __future__ import annotations
from pconfigs import pconfig, pdefaults, Pinned, pproperty

@pconfig
class BaseConfig:
mode: str

pdefaults += BaseConfig(
mode='train',
)

@pconfig
class DerivedConfig(BaseConfig):
mode: Pinned[str]

@pproperty
def mode(self) -> str:
return 'eval'

pdefaults += DerivedConfig(
mode=Pinned,
)

print(DerivedConfig(BaseConfig()).mode) # raises ValueError ✗ — proposed: 'eval'
```

And when the pinned field uses a plain `Pin(value)` default:

```python
@pconfig
class DerivedConfig(BaseConfig):
mode: Pinned[str]

pdefaults += DerivedConfig(
mode=Pin('eval'),
)

print(DerivedConfig(BaseConfig()).mode) # raises ValueError ✗ — proposed: 'eval'
```

**Current workaround**

Pass the pinned default explicitly at the call site.

For the `@pproperty` case:
```python
config = DerivedConfig(
BaseConfig(),
mode=Pinned,
)
# ✓ returns 'eval'
```

For the `Pin(value)` case:
```python
config = DerivedConfig(
BaseConfig(),
mode=Pin(pdefaults(DerivedConfig).mode),
)
# ✓ returns 'eval'
```

Start by tracing copy construction from BaseConfig to DerivedConfig through the pconfig, Pinned, pdefaults, pproperty, and Pin entry points shown in the examples. Reproduce both the @pproperty and Pin(value) cases, then verify that DerivedConfig(BaseConfig()).mode uses the derived pdefaults value without requiring an explicit pinned argument.

Written by the indexing model from the issue text.