vendor: use Composer for HTML block conversion

[chubes4]

Summary

• Promotes chubes4/html-to-blocks-converter to a runtime Composer dependency instead of a dev-only dependency bundled through committed vendor_prefixed/ snapshots.
• Stops tracking generated vendor_prefixed/ output in source while keeping composer build available for release/plugin artifacts.
• Updates the lockfile so source consumers resolve the HTML-to-blocks package through Composer.

Testing

• composer validate --strict
• composer build
• php tests/smoke-scoped-h2bc-hooks.php

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafting and applying the Composer dependency cleanup, running local verification, and preparing this PR description. Chris remains responsible for review and merge.