Variable blockquoteRuleConst

Block rule for > prefix blockquotes.

Parsing strategy:

1. Collect consecutive lines that begin with BLOCKQUOTE_MARKER at line start.
2. For each line, record the depth (number of > chars, zero-indexed) and parse the inline content after the mandatory space.
3. Lines missing the required space are consumed but produce no output.
4. Feed the flat depth list into processDepths to build a nested tree.
5. Recursively convert the tree into nested blockquote container elements via buildBlockquoteElement().