✨ Prefer markly for Markdown; stop forcing other Markdown backends

[pboling]

Why

• Wrong :auto order: markdown-merge picked an unspecified (:auto) backend in BACKEND_REFERENCES order: kreuzberg-language-pack, then commonmarker, markly, kramdown. The language pack is always registered, so the native parsers never got a chance. In kettle-jem's bundle today, Markdown::Merge.resolve_backend(nil) returns "kreuzberg-language-pack". Markly is the best Markdown parser we have and should come first.
• Forced dependencies: kettle-jem declared commonmarker-merge and kramdown-merge as runtime dependencies, although it loads only markdown/merge and ast/crispr/markdown/markly. Every gem that bundles kettle-jem therefore compiles commonmarker. That native extension does not build on ruby-head: magnus 0.8 is missing Ruby 4.1's handle_weak_references, and the fix sits unreleased on magnus main. kettle-jem-appraisals' ruby-head job has failed in setup this way since 2026-08-08.

What

• markdown-merge
  • New BACKEND_PREFERENCE (markly, commonmarker, kramdown, kreuzberg-language-pack) drives :auto resolution. An explicit backend and TreeHaver.current_backend_id still win, and the language pack stays the final fallback.
  • Optional backend gems are still loaded only if installed (require … rescue LoadError), now markly first.
  • BACKEND_REFERENCES and available_markdown_backends keep their order, which the shared structuredmerge-fixtures slice-195 fixture defines for every language implementation.
• kettle-jem
  • Drops the commonmarker-merge and kramdown-merge runtime dependencies. They are still used when a project installs them.
  • Removes both gems from the local sibling list in Gemfile, from gemfiles/modular/templating_local.gemfile, and from the templating_local.gemfile.example template.
  • Gemfile.lock was updated with bundle lock, using the env the committed lock is generated with. The diff only removes commonmarker-merge, kramdown-merge, commonmarker (all platforms) and rb_sys.

Verification

• markdown-merge: full suite passes (46 examples, 0 failures). RuboCop Gradual passes once the ✨ kettle-jem: security floors from kettle-rb; drop dead appraisal helpers #69 Gemfile fix is present.
  • .rubocop_gradual.lock was force-updated with rubocop-gradual --force-update, with that fix applied locally. Only the lib/markdown/merge.rb entries change (line shifts from the new constant).
  • Until ✨ kettle-jem: security floors from kettle-rb; drop dead appraisal helpers #69 merges, this branch alone still shows the Gemfile Bundler/DuplicatedGem that is already on main.
• Runtime check in kettle-jem's bundle (loading ast/crispr/markdown/markly and markdown/merge): Markdown::Merge.resolve_backend(nil) was "kreuzberg-language-pack" before and is "markly" now. available_markdown_backends keeps its order.
• kettle-jem full suite against this branch's sibling gems (STRUCTUREDMERGE_DEV pointing at the branch, K_JEM_TEMPLATING=true): 686 examples, 0 failures.
• Unaffected cross-language fixture: the structuredmerge-fixtures slice-195 fixture still passes (fixtures_integration_spec).

🤖 Generated with Claude Code

https://claude.ai/code/session_01P8aveNcdgPobyMrq8DkD9r