Phase 7.16: add alpha documentation checkpoint

[josiahking]

Summary

Add the EvolvePHP 2 Alpha documentation checkpoint before the first Alpha release.

This adds:

• external-developer Alpha getting-started guidance
• application-foundation documentation
• modernization and Bridge guidance
• Alpha status, compatibility and limitations documentation
• a contributor-facing CONTRIBUTING.md
• draft 2.0.0-alpha.1 release notes
• updated support, skeleton and root documentation
• documentation-policy coverage for the Alpha checkpoint

Boundaries

This is a documentation-only slice.

It does not:

• change runtime or package source
• change Composer manifests or the lock file
• change package/release topology
• publish packages
• create a release or tag
• claim public Packagist installation
• claim production readiness

The draft Alpha release notes remain explicitly unreleased.

Validation

Validated on PHP 8.4 with:

• focused Alpha documentation tests
• complete Documentation suite
• Architecture + Documentation tests
• PHPStan
• Deptrac
• PHP-CS-Fixer
• strict Composer validation
• release package inventory validation
• root quality
• public wording and unsupported-install-claim checks

Package split, prerelease-consumer, skeleton create-project, supply-chain and
benchmark validators are intentionally not applicable to this documentation-only
change. They remain available for the actual Alpha release gate.