Fix EAC-CPF namespace handling in XPath queries

[Copilot]

Addresses namespace inconsistency in traject_config_eac_cpf.rb where XPath queries mixed prefixed and unprefixed element names. Since EAC-CPF documents use a default namespace (xmlns="urn:isbn:1-931666-33-4"), unprefixed XPath queries fail to match elements.

Changes

• Define namespace constant: EAC_NS = { 'eac' => 'urn:isbn:1-931666-33-4' }
• Standardize all XPath queries: Use eac: prefix consistently across all 24 queries
• Remove fallback queries: Eliminate unprefixed fallback logic that would never execute

Before:

# Mixed approaches - inline mapping with unprefixed elements, or no namespace
entity = record.xpath('//cpfDescription/identity/entityType', 
                     'eac-cpf' => 'urn:isbn:1-931666-33-4').first
entity ||= record.xpath('//identity/entityType').first  # Fallback never matches

After:

# Consistent prefix with shared namespace constant
entity = record.xpath('//eac:cpfDescription/eac:identity/eac:entityType', EAC_NS).first

This ensures reliable extraction from ArchivesSpace EAC-CPF exports.

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