Add Doctor diagnostic foundation

[josiahking]

Summary

Adds the runtime-neutral foundation for Evolve Doctor diagnostics in evolvephp/core, including deterministic diagnostic results and the first PHP runtime checks.

What changed

• Added DoctorCheck for composable diagnostic checks.
• Added immutable DoctorFinding, DoctorStatus, and DoctorReport result models.
• Added DoctorRunner with deterministic ordering, duplicate identifier protection, and result validation.
• Added a PHP version compatibility check with PHP 8.4 as the minimum supported runtime.
• Added a caller-supplied PHP extension check with deterministic missing-extension reporting.
• Added unit coverage for diagnostic validation, runner behavior, PHP versions, and PHP extensions.
• Updated Core documentation with the current Doctor capabilities and limitations.
• Updated the Core source inventory used by architecture validation.

Validation

• Full test suite: 535 tests, 2038 assertions
• Static analysis passed
• Dependency-boundary checks passed
• Coding-style checks passed
• Security and licence checks passed
• Release package validation passed
• Package split validation passed
• Consumer validation passed

Package impact

The new diagnostic foundation is contained in evolvephp/core. No package dependencies or Composer manifests were changed.