Living Dictionaries support: LD-shaped round-trip test + export docs

Background

SIL is investigating LIFT export for Living Dictionaries (sillsdev/TheCombine#4332); if it proceeds, python-sil-lift would be the conformance validator, a Python writer for a converter route, and the reference for conformant output. The writer/validator/CLI already cover the constructs involved — this issue adds the two things still missing here: a test proving the LD field model round-trips, and export docs tailored to the LD case.

Tests

• Full LD-shaped entry round-trip. One entry exercising every construct the LD→LIFT mapping uses, together: a multi-orthography lexical-unit; a Pronunciation with a speaker-label forms entry (The Combine's Speaker: <name> convention) plus media; a Sense with grammatical-info, gloss, definition, an Example + Translation, an illustration (URLRef with a caption label), a semantic-domain-ddp4 trait, and LD-only data carried as <field>/<trait>. Build → save → reload → assert the values survive and iter_problems() is clean. The model already supports all of these, so this is verification, not new code.
• (Optional, once a real sample is available) add a representative Living Dictionaries export as a corpus fixture (with provenance) and validate it end-to-end.

Docs

• Semantic-domain representation for LD — in the Producing conformant LIFT guide (docs/en/guides/lift-export-interop.md), document how to emit LD's ~69-key vocabulary as its own named range in .lift-ranges (lossless, self-describing), with a short example.
• LD → LIFT field-mapping reference — a concise summary (MultiString → <form lang>, audio/speaker labels, sense photos as <illustration>, LD-only fields as <field>/<trait>) so an exporter author has one place to look; can build on the existing Building a LIFT export from scratch example (docs/en/guides/build-export.md).

Non-goals

• Any LD exporter (would live in Living Dictionaries / a converter, not here).
• Bundling the DDP4 taxonomy.

Start with the existing writer, validator, and CLI tests, then use the LD-shaped construct list in this issue to assemble the round-trip case and check iter_problems(). Update docs/en/guides/lift-export-interop.md with the named semantic-domain range example and use docs/en/guides/build-export.md as the basis for the field-mapping reference. Done means the values survive build, save, and reload, validation is clean, and both LD export topics are documented.