SPIKE-1: Confirm MDTranslator xlink resolution behavior

**What:** Many federal ISO 19139 records use `xlink:href` to reference external endpoints for contact information, coordinate reference system definitions, and responsible party data. The Census Bureau WAF records are a confirmed example -- contact, originator, and CRS are all xlink-referenced to NGDC (NOAA) docucomp endpoints rather than inline values.

The harvester's `download_file()` function fetches raw XML and passes it to MDTranslator. It is not known whether MDTranslator dereferences xlinks internally before parsing.

**Investigate and document:**
- Submit the Census concity ISO record and other test WAFs to the MDTranslator service and inspect the output:
- Reader: `iso19115_2_datagov`
- Writer: `dcat_us`
- Does the output contain populated contact, publisher, and CRS fields, or are they empty?
- If empty: are they empty because xlinks were not resolved, or because MDTranslator does not map those fields at all?
- Check which fields in the output correspond to `fill_placeholders` defaults vs. values derived from the source record
- Test other ISO records that have inline (non-xlink) contact values to compare output

**Done When:**
- Findings documented with MDTranslator input/output for the test records
- Clear answer on whether xlinks are resolved
- If xlinks are not resolved, follow-up implementation ticket written (see IMPL-5)

**Notes:**

The NGDC xlink endpoints referenced in the Census record are:
- Contact: `https://www.ngdc.noaa.gov/docucomp/1df27e57-4768-42de-909b-52f530601fba`
- Originator: `https://www.ngdc.noaa.gov/docucomp/ddd21bfb-2229-465b-95b2-bee36200b0e5`
- CRS (NAD83): `https://www.ngdc.noaa.gov/docucomp/65f8b220-95ed-11e0-aa80-0800200c9a66`

These endpoints may or may not be publicly accessible; test reachability first.