fix(whatsapp): preserve literal text during formatting

[luksch42]

Summary

Fixes #908.

Outgoing WhatsApp Markdown such as (~80 % easy) currently becomes (\~80 % easy), leaving a visible backslash in the delivered message. Render literal text, bold, and strikethrough directly from the Markdown AST so literal tildes and intentional backslashes are preserved, and formatting markers inside inline and fenced code remain unchanged.

Expose typed serialization handlers in stringifyMarkdown and provide lookahead markers to avoid repeated subtree serialization for deeply nested formatting. Includes regression tests, patch changesets, and documentation updates.

Test plan

• pnpm validate passes (knip, lint/format, typecheck, tests, and build).
• pnpm konsistent passes.
• pnpm changeset status recognizes the patch changesets.
• Regression coverage includes literal tildes, intentional backslashes, escaped markers, inline/fenced code, lists, and nested formatting, including a deep-nesting performance check.

Checklist

• All commits are signed and verified
• All commits are signed off for the DCO (git commit -s)
• pnpm validate passes
• Changeset added
• Documentation updated

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