Harden php-transformer release metadata

[chubes4]

Summary

• add a php-transformer GitHub Actions workflow for Composer install, strict metadata validation, and package tests
• add a Composer branch alias for the pre-release trunk line while keeping released versions tag-derived
• clarify tagged Packagist/subtree release readiness docs and the no replace/provide policy for downstream wrappers

Verification

• composer validate --strict
• composer test
• git diff --check

Operator action remaining

• After this PR is merged and downstream merge candidates no longer require path repositories or unpublished branches, run the release dry-run from the merged package ref, create the first 0.1.0 tag/subtree or Packagist package root for php-transformer, and publish/index it. This PR does not tag, publish, create a release, or deploy.

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 + OpenCode
• Used for: Inspecting package release metadata, drafting focused Composer/docs/CI changes, running verification, and preparing this PR.