[API Proposal]: CopyFileOrDirectory

### Background and motivation

`ChainablePathExtensions` offers `CreateDirectoryRecursively`, `DeleteFileOrDirectory` and `MoveFileOrDirectory`, but not copy. Copying is at least as common as moving in build scripts and tests, and a recursive directory copy is genuinely tedious to write correctly by hand (creating intermediate directories, preserving relative structure, deciding what to do about existing files). The gap is conspicuous next to the existing move support, including its collection overload.

### API Proposal

```C#
namespace Pathy
{
public static class ChainablePathExtensions
{
public static void CopyFileOrDirectory(
this ChainablePath sourcePath,
ChainablePath destinationDirectory,
string newName = null,
bool overwrite = false);

public static void CopyFileOrDirectory(
this IEnumerable sourcePaths,
ChainablePath destinationDirectory,
bool overwrite = false);
}
}
```

The signature deliberately mirrors the existing `MoveFileOrDirectory` overloads.

### API Usage

```C#
// Copy a single file, optionally renaming it
(ChainablePath.Current / "appsettings.json")
.CopyFileOrDirectory(ChainablePath.Temp / "config", newName: "appsettings.local.json");

// Copy a whole directory tree
(ChainablePath.Current / "templates")
.CopyFileOrDirectory(outputDirectory, overwrite: true);

// Copy a matched set of files
(ChainablePath.Current / "src")
.GlobFiles("**/*.md")
.CopyFileOrDirectory(docsDirectory);
```

### Alternative Designs

Separate `CopyFile` and `CopyDirectory` methods. The library has already chosen the combined `FileOrDirectory` naming for delete and move, so following it keeps the API predictable.

### Risks

Copying a directory into itself or into one of its own descendants must be detected, otherwise the operation never terminates. The default for `overwrite` should match whatever `MoveFileOrDirectory` does today, and the behaviour when the destination directory does not yet exist has to be defined (most likely: create it).

Start by reading ChainablePathExtensions and the existing MoveFileOrDirectory overloads, including their overwrite and destination handling. Define the matching single-path and collection copy behavior, recursive directory handling, self-descendant protection, and destination creation, then verify the API covers the usage examples and documented edge cases.

Written by the indexing model from the issue text.