Fix StackOverflowError in recursive AST conversion

[elevenfive]

Summary

The convert() function in AstNodeConvert.kt converts a CommonMark Node tree into an AstNode tree using recursion in two directions — the sibling chain (node.next) and the child chain (node.firstChild). Both recurse without bound, causing StackOverflowError crashes in production on Android's ~1MB default thread stack when documents contain many sequential blocks or deeply nested structures.

Crashlytics stack trace:

java.lang.StackOverflowError: stack size 1037KB
  at com.halilibo.richtext.markdown.node.AstNode.<init> (AstNode.kt:4)
  at com.halilibo.richtext.commonmark.AstNodeConvertKt.convert (AstNodeConvert.kt:170)
  at com.halilibo.richtext.commonmark.AstNodeConvertKt.convert (AstNodeConvert.kt:174)
  at com.halilibo.richtext.commonmark.AstNodeConvertKt.convert (AstNodeConvert.kt:175)
  ... (hundreds of frames)

Changes:

• Replaced the recursive convert() with an iterative implementation: siblings are traversed in a while loop, and child processing is pushed onto an explicit ArrayDeque stack
• Extracted the node-type mapping when block into a private convertNodeType() helper (no logic changes)
• The function signature and external behavior are unchanged — the produced AstNode tree is identical (all AstNodeLinks — parent, firstChild, lastChild, next, previous — are wired up the same way)

Tests added:

• Tree link correctness for documents with siblings (parent/previous/next/lastChild)
• Tree link correctness for nested structures (blockquote → paragraph → text)
• 2,000 sibling paragraphs complete without overflow
• 500 levels of nested blockquotes complete without overflow
• Constrained-stack proof: builds a 5,000-sibling CommonMark tree and runs convert() on a thread with a 256KB stack — first proves that equivalent-depth recursion overflows, then proves the iterative convert() handles it

Test plan

• ./gradlew :richtext-commonmark:allTests passes (6 tests, 0 failures)
• Verified the old recursive implementation fails 2 of the new tests (StackOverflowError on 2,000 siblings; assertion failure on the constrained-stack test)
• Existing test (image without title) continues to pass unchanged

🤖 Generated with Claude Code

[mzavislak-inflection]

cc @halilozercan can we approve the workflow to test?