fix(block): HTML block start outside a list item closes the list

[inchang-ing]

Fixes #434

Problem

A line that starts an HTML block (any type) on a line under-indented for the list item's content was consumed as a lazy continuation of the item's paragraph:

## Opts

- a
<br>
## Next

rendered as <li>a\n<br></li> in markdown-it-py (and in the JS original), while cmark and commonmark.js close the list and open the HTML block at the outer level — because such a line can neither be item content nor a lazy continuation of it (it starts a new block). Flipping the type-7 "can terminate paragraph" flag is not a fix: it would let <br> interrupt a top-level paragraph, where all implementations agree it must not.

Change

• html_block.py: new html_block_starts(state, line) helper — detects whether a line begins an HTML block of any type (same sequence table as the rule).
• paragraph.py: while scanning continuation lines, end the paragraph when the next line starts an HTML block and is under-indented for the current container (sCount < blkIndent). The list rule then sees the line, finds no item marker, and closes the list, so the line is offered to the outer containers — reproducing cmark's container walk. At the same container level a type-7 sequence still does not interrupt the paragraph (the terminator rules are unchanged), and plain text at outer indentation remains a lazy continuation.

Verification

• The issue's minimal input now renders exactly the cmark output: <h2>Opts</h2><ul><li>a</li></ul><br>\n## Next (the type-7 block swallows ## Next up to a blank line, as cmark does).
• Four boundary cases are pinned in tests/test_html_block_list.py: HTML block at the item's content indent stays a lazy continuation; a top-level paragraph still continues through <br>; plain text at outer indentation is still a lazy continuation; a type-6 block (<div>) still closes the list as before.
• Full suite: failure list is byte-identical to the pre-change baseline (74 pre-existing environment failures, git stash diffed — zero regressions; 958+ passed).
• ruff check and ruff format clean on touched files.

Note

The JS markdown-it behaves the same way as this library currently does, so this change intentionally diverges from upstream JS toward the cmark/commonmark.js behavior documented in the issue; happy to align on whatever the maintainers decide.

Disclosure

This PR was prepared with AI assistance (ZCode/GLM, orchestrated via WorkBuddy).