Phase 7.14: add PHP 7 legacy remote client

[josiahking]

Summary

• add an isolated PHP 7.4-compatible Remote Bridge client under compat/legacy-http-client
• preserve the existing Remote Bridge protocol v1 wire contract
• keep delegation header filtering separate from protocol encoding
• provide bounded cURL transport behavior with TLS verification, redirects disabled, explicit timeouts, and conservative uncertain-outcome handling
• preserve the normal 12-package PHP 8.4 release graph
• add dedicated real-PHP-7.4 CI coverage with JSON and cURL runtime checks

Validation

• standalone legacy client: 12 tests passed
• compatibility PHPStan: clean
• root quality: 963 tests / 3997 assertions / 1 skipped
• deterministic package split validation: passed
• prerelease consumer validation: passed
• skeleton create-project validation: passed
• supply-chain validation: passed
• release inventory: 12 packages
• committed-ref gate: 11 pass / 0 fail

Real PHP 7.4 execution is intentionally proven by the dedicated CI job in this PR.