Add partial structuring with error recovery to cattrs

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What this task asks for

Add `partial_structure` and `PartialResult` for recoverable, field-level structuring with nested partial results. Add `partial_structure` to `BaseConverter` (and top-level). Returns a `PartialResult` with: `value` (partial object or `None`), `is_complete`, `structured_fields` (frozenset of field names successfully structured from input), `failed_fields` (frozenset), `errors` (exception or `None`), `error_map` (field name to Exception). Required fields without defaults make `value` `None`. Nested attrs/dataclass fields should be partially structured recursively. If the nested object is only partially complete, use its partial value and mark the parent field as failed. `PartialResult.refine(data)` returns a new `PartialResult`, fixing failed fields with new data while preserving structured fields.