[Clippy] feat: add RegisterCustomHandler extensibility API to DocumentAssembler

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🤖 This PR was created by Clippy, an automated AI assistant.

Closes #66

Summary

Adds a thread-safe, static extensibility API to DocumentAssembler that allows users to register custom content-control directives without forking the library.

New Public API

// Delegate type for custom handlers
public delegate object? CustomAssemblerHandler(XElement directive, XElement data, OpenXmlPart part);

// Register a custom directive handler (with optional XSD schema for validation)
DocumentAssembler.RegisterCustomHandler(string elementName, string? schemaXsd, CustomAssemblerHandler handler);

// Remove a previously registered handler
DocumentAssembler.UnregisterCustomHandler(string elementName);

Usage example — replace a <#(MyGreeting/)#> directive in a template:

DocumentAssembler.RegisterCustomHandler("MyGreeting", schemaXsd: null, (directive, data, part) =>
{
    var name = (string?)data.Element("Name") ?? "World";
    return new XElement(W.r, new XElement(W.t, $"Hello, {name}!"));
});
var result = DocumentAssembler.AssembleDocument(templateWml, dataXml, out var hasErrors);
DocumentAssembler.UnregisterCustomHandler("MyGreeting");
```

The handler receives the directive `XElement`, the current data context `XElement`, and the `OpenXmlPart`. It can return:
- an `XElement` / `XNode` (replaces the directive in the document tree)
- a `string` (wrapped in a `(w:r)(w:t)` run)
- `null` (silently removes the directive)
- throwing an exception maps to a `TemplateError` in the output document

## Design Notes

- **Thread safety**: uses `ConcurrentDictionary` for lock-free reads on the hot path; write operations are also thread-safe.
- **Guard**: `RegisterCustomHandler` throws `ArgumentException` if the element name conflicts with a built-in PA directive (`Content`, `Table`, `Repeat`, `Image`, etc.).
- **XSD validation**: pass an XSD string to validate directive attributes at template-parse time; pass `null` to skip schema validation for that directive.
- **Block-level promotion**: custom directives are automatically included in `ForceBlockLevelAsAppropriate` so block-level substitutions (e.g. replacing with a table or heading) work correctly.
- **Alias recognition**: custom element names are added to `s_aliasList` so content controls with a matching `(w:alias)` are recognized as template directives.

## Tests Added (DA500–DA504)

| Test | What it verifies |
|------|-----------------|
| DA500 | Basic invocation: handler fires and its return value replaces the directive |
| DA501 | Exception in handler maps to a `TemplateError` in the output |
| DA502 | Handler returning `null` silently removes the directive element |
| DA503 | After `UnregisterCustomHandler` the element is treated as an invalid directive |
| DA504 | Custom XSD schema rejects directive with invalid attributes |

## Test Status

```
dotnet test --project Clippit.Tests/
total: 1182  succeeded: 1181  failed: 0  skipped: 1
✅ All tests pass

dotnet csharpier check . → ✅ all 204 files formatted correctly

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