Phase 7.15: add modernization cutover acceptance

[josiahking]

Summary

• add deterministic modernization cutover acceptance coverage for billing.invoice-summary
• prove equivalent execution through Laravel embedded, Symfony embedded, and legacy Remote Bridge paths using the same Evolve capability handler
• exercise existing Audit and Adoption declarations for pre-cutover evidence, Evolve ownership after cutover, rollback evidence, and legacy retirement
• extend the isolated legacy remote client fixture and standalone runner with the representative modernization vector

Scope

This is an acceptance/evidence slice. It does not add a migration engine, automatic cutover, database synchronization, rollback execution, deployment orchestration, or a new runtime abstraction.

Validation

• modernization acceptance: 3 tests, 61 assertions
• legacy remote client standalone: 13 tests
• Architecture + Documentation: 243 tests, 14,195 assertions
• PHPStan: clean
• Deptrac: 0 violations / 0 uncovered
• PHP-CS-Fixer: clean
• Composer validation: clean
• release package inventory: 12 packages
• deterministic package splits: passed
• prerelease consumer validation: passed
• skeleton create-project validation: passed
• supply-chain validation: passed
• root quality: passed

The dedicated pull-request CI remains responsible for the real PHP 7.4 legacy-client runtime proof.