Expose and document Encodable.register - #772
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The encoding registry was designed to be extensible so that a type the library serializes badly, or not at all, can be given an encoding without a library-level fix. That hook was only reachable as `TypeToPydanticType.register` on the internal handler class, so the escape hatch was effectively undiscoverable from the public interface. `register` is now a classmethod on `Encodable`, documented with a doctest that starts from the failure a user actually hits and ends with the type encoding, decoding and schematizing. It takes a `TypeForm` rather than a `type`, since the registry also accepts unions and typing special forms, and returns a decorator that preserves the function it decorates. The internal registrations keep calling `TypeToPydanticType.register`; the library registering into its own handler class is the right layering. The developer-facing pointer in `_NoEncoding`'s error message still names the internal class, deliberately: #770 deletes that block outright, so editing it here would buy nothing but a merge conflict. `Encodable` stays a `TYPE_CHECKING`-only alias, so `@Encodable.register` in typed code needs `# type: ignore[attr-defined]`, as the example shows. That is forced, not incidental: mypy resolves a name as either a generic alias (usable in annotations) or an object with attributes, never both. Replacing the alias with a class of any shape -- plain, generic, Protocol, or a `TypeForm`-typed metaclass `__getitem__` -- makes `register` check but breaks `field: Encodable[T]`, and mypy ignores `__class_getitem__` entirely, so no subscript signature can rescue it. Under that ignore the call is `Any`, so the signature documents rather than checks. Closes #769 Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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Resolves #769