Harden php-transformer release readiness

[chubes4]

Summary

• Add Composer package authorship metadata for the php-transformer package.
• Add a Composer-run package install proof that mirrors php-transformer into a clean throwaway Composer project with symlink disabled and verifies autoloaded transformer output.
• Update README and packaging docs to distinguish reviewer-safe checks from operator-only release/tag/publish steps.
• Refresh install proof docs to avoid stale branch-specific constraints and document the new composer test:packaging command.

Findings

• Composer metadata validated cleanly, but package authorship metadata was still missing.
• Existing CI ran package tests from the working tree only; it did not prove Composer could install the subtree package root as a mirrored package.
• Install proof docs still referenced an old review branch constraint and local observed output.
• Release docs mixed reviewer preflight checks with operator-only tag/release/package publication actions.

Verification

• composer validate --strict from php-transformer/
• composer test from php-transformer/
• git diff --check

Residual release blockers / operator-only checklist

• No tags, GitHub releases, Homeboy release commands, Packagist publication, or package publishing were run in this PR.
• Operator must choose the exact merged commit that should own 0.1.0.
• Operator must run the Homeboy dry-run from the accepted merged commit.
• Operator must create the package tag from the accepted release path.
• Operator must publish or connect the Packagist/subtree-split package root if that is the chosen distribution path.
• Operator must update downstream wrapper/product PRs from review-only constraints to tagged constraints before merge.
• Operator must decide whether GitHub Releases are part of the first package publication path.

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Auditing release-readiness surfaces, drafting package-install coverage, updating documentation, and running verification.