Preserve unmodeled <file>/<trans-unit> (and inline) attributes on round-trip

`Xliff::File` models only `original`, `source-language`, `target-language`, and `datatype`; `Xliff::Entry` models only `id` and `xml:space`. Every *other* attribute — on ``, ``, ``, ``, and `` — is silently dropped on parse and never re-emitted.

Confirmed dropped from a fully standard-root document (all legal XLIFF 1.2 attributes):

- ``: `product-name`, `date`, `tool-id`, …
- ``: `approved`, `maxwidth`, … (`approved` marks a unit as reviewed — core translation-workflow state)
- ``: `state`, `xml:lang`; ``: `xml:lang`; ``: `from`, `priority`

So `Bundle.from_string(x).to_s != x` for any document carrying attributes beyond the modeled set, even when its root is fully standard. This is the same "fixed model drops the rest" family as #16 (root attributes), #17 (``/``), and #18 (header namespaces).

## Proposed fix

On parse, capture each element's unmodeled attributes (everything beyond the known set) and replay them on write — bounded: read `xml.attribute_nodes`, keep the ones the model doesn't already own, re-emit them in `to_xml`.

## Scope

Split out of the #15 review sweep. Xcode's own exports use only the modeled attributes, so this isn't required for that use case, but it's silent data loss for general XLIFF 1.2 and undercuts the byte-identical round-trip claim. The README documents the limitation until this lands.

Locate the parse and to_xml implementations for Xliff::File, Xliff::Entry, and the source, target, and note elements; start by tracing how xml.attribute_nodes are currently handled. Preserve attributes outside each model's known set and verify that standard XLIFF 1.2 documents with the listed attributes round-trip without losing them.

Written by the indexing model from the issue text.