Add multi-phase ESPI schema compliance plan and reference diagrams (#49)

Addresses Issue #28

  This commit adds a comprehensive 26-phase plan to review and update all
  Usage and Customer domain entities to ensure compliance with NAESB ESPI 4.0
  schema (espi.xsd and customer.xsd) element sequences.

  Added files:
  - MULTI_PHASE_SCHEMA_COMPLIANCE_PLAN.md: Detailed 26-phase implementation
    plan with tasks for entities, DTOs, mappers, repositories, services,
    Flyway migrations, and tests
  - ESPI Usage.pdf: Atom link relationship diagram for espi.xsd main resources
  - ESPI Usage Summary Classes.pdf: Atom link diagram for summary classes
  - Retail Customer Resource Atom Link Usage.pdf: Atom link relationships
    for customer.xsd resources

  The plan processes entities in dependency order:
  - Phases 1-16: Usage domain entities (espi.xsd)
  - Phases 17-26: Customer domain entities (customer.xsd)

  Each phase includes:
  1. Entity field order verification against XSD
  2. DTO updates with JAXB annotations
  3. Bidirectional MapStruct mapper updates
  4. Repository query optimization
  5. Service method compliance review
  6. Flyway migration script updates
  7. Comprehensive testing with XML marshalling/unmarshalling
  8. Git workflow (branch, commit, push, PR)

  Key corrections applied:
  - MeterReading has no child elements, only Atom links
  - ServiceLocation references UsagePoint via cross-stream href URL
  - Bidirectional Atom link relationships properly documented
  - Customer domain entity relationships corrected per schema diagrams

Author: dfcoffin