Fix #28: Add JSON payload builder for DetClassification + preferred_name alias fix

[ronaldtse]

Summary

Closes #28. Two fixes in one commit because the second was discovered
while writing the spec for the first.

Fix 1 — payload builder (the issue)

Register Opencdd::DetClassification in PAYLOAD_BUILDERS and add
a det_classification_node method following the same pattern as
view_control_node / value_list_node / list_of_unit_node. DET
classification entities have standard fields (code, preferred_name,
definition, version, revision) that flow through entity_payload;
no type-specific post-processing is needed.

Before:

ArgumentError: No JSON payload builder for Opencdd::DetClassification

After: 163 entities serialize into the browser JSON.

Fix 2 — preferred_name property-ID alias (drive-by)

The alias target for IECCDD_001_C0002 was MDC_P004_1
(preferred_name_localized), but the field DSL's
field :preferred_name, "MDC_P004" reads from MDC_P004
(preferred_name base). Result: every DET classification entity's
preferred_name was nil even after import.

The per-language entries (IECCDD_001_C0002.en → MDC_P004_1.en, etc.)
were dead code: canonical_id splits the language suffix off before
the lookup and reapplies it after, so only the base entry is
consulted. Removed them; changed the base to MDC_P004 so the
output is MDC_P004.<lang> — what the preferred_name DSL finds.

Test plan

• bundle exec rspec — 974 examples, 0 failures (up from 962)
• New det_classification context in
  spec/exporters/per_entity_json_spec.rb asserts type, irdi,
  code, and preferred_name
• Existing det_classification_aliased_spec still passes (the
  end-to-end import spec — verified the alias change doesn't
  break the 163-entity count)
• Manual verification needed in data-private:
  bash cd ../data-private bundle exec rake browser:build_from_export[iec-61360-4] python3 -c " import json db = json.load(open('data/iec-61360-4/database.json')) det = [e for e in db if e.get('type') == 'det_classification'] print(f'DET classification entities in browser JSON: {len(det)}') print(f'Sample: {det[0][\"code\"]} - {det[0].get(\"preferred_name\")}'[:80]) "
  Expected: 163 entities, first code="A11" with non-null preferred_name.