Add SI detail page scraping (structured register content)

[philipp-gecko]

Summary

• Adds --detail / -det CLI flag that fetches the SI (Strukturierter Registerinhalt) XJustiz XML document for the first search result
• Parses the XML into structured fields: company name, address, legal form, share capital, directors, prokura holders, business purpose, representation rules, and register entry history
• Works with both text and JSON (-j) output modes
• No new dependencies — uses stdlib xml.etree.ElementTree

Example

$ python handelsregister.py -s "Gecko Two GmbH" -so exact --detail -f

name: Gecko Two GmbH
court: Saxony   District court Leipzig HRB 32007
...

--- Detail (SI) ---
Name: Gecko Two GmbH
Legal Form: Gesellschaft mit beschränkter Haftung (GmbH)
Address: Ritterstraße 42, 04109 Leipzig
Capital: 35333.00 EUR
Directors:
  Christopher Siebenhüner (einzelvertretungsberechtigt)
Prokura:
  Peter Schlangen (Einzelprokura)
  Tom Potutschek (Einzelprokura)
...

Test plan

• Unit test test_parse_si_detail — parses XJustiz XML fixture (no network)
• Integration test test_fetch_detail — searches "Gecko Two GmbH" and verifies SI fields
• Existing tests pass without modification (added detail=False to argparse Namespace)
• --detail flag is opt-in, no behavior change without it

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