fix: unify default engine with createProcessor for consistent blockquote rendering

[jaredwray]

Summary

• Unifies the default engine with createProcessor — the static default engine (line 51) was hand-coded and inconsistent with createProcessor, missing remarkGithubBlockquoteAlert and having remarkMath in the wrong plugin phase (after remarkRehype instead of before). The constructor now always uses createProcessor to build the engine.
• Fixes remarkMath plugin ordering — remarkMath is a remark plugin (mdast phase) but was placed after remarkRehype (the bridge to hast). It's now correctly placed before remarkRehype, with rehypeKatex remaining in the rehype phase.
• Adds test coverage for blockquote + list rendering — three new tests covering the exact scenario from issue Content inside blockquotes don't follow markdown rules. #443 (lists inside blockquotes with emoji), lists without blank separator lines, and sync rendering.

Closes #443

Test plan

• All 176 existing tests pass
• 3 new blockquote + list tests pass
• pnpm build succeeds
• pnpm test passes with 100% statement/function/line coverage

https://claude.ai/code/session_01EtCwTREZoVsyLxnPKp6Fzu

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a2b0867) to head (1c02d50).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #444   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          470       472    +2     
  Branches       105       109    +4     
=========================================
+ Hits           470       472    +2     

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[gemini-code-assist]

Code Review

This pull request refactors the Writr class to centralize the initialization of the unified processor and improves the handling of Markdown plugins like remarkToc and remarkMath. Specifically, the engine property is now initialized once in the constructor using a dedicated createProcessor method, and new tests have been added to ensure correct rendering of lists within blockquotes. A review comment suggests improving type safety by replacing the any type on the engine property with a more specific type from the unified package.