Escape leading colon markers in HtmlToDjot + roundtrip property net

[dereuromark]

Follow-up to the HtmlToDjot escaping work (#202, #203).

Problem (G1)

A paragraph of external HTML beginning with a colon was re-parsed as a different block on the next Djot parse:

<p>: just text</p>     ->  : just text     ->  <dl><dt>just text</dt>...   (paragraph -> definition list)
<p>::: not a div</p>   ->  ::: not a div   ->  <div class="not a div"></div>   (text DESTROYED into a class)
<dd>: value</dd>       ->  ...             ->  nested <dl> corruption

The ::: case is the worst: the content is captured into the div class name and the text is lost.

This is the same class as the leading -, #, > and 1. markers already handled; escapeLeadingBlockMarker simply did not cover :.

Fix

Add a leading colon to the block-marker escaping. Escaping the first colon neutralizes both a definition-list item (: term) and a fenced div (::: name), so the text stays literal: \: just text, \::: not a div.

Property net (G4)

Adds a generated round-trip property test that, for externally-authored HTML, sweeps Djot-significant leading tokens (- + * # > : ::: 1. 1) --- ~~~ ``` | ...) and inline markers across paragraph and inline-wrapped (<span>) contexts, asserting:

• no block is injected (no list, heading, blockquote, div, definition list, rule, table, or section), and
• the text survives (alphanumeric content preserved, so data loss like the ::: case is caught).

This sweep is what surfaced the colon gap, and it guards the whole "a literal marker silently became a block" class going forward instead of one example at a time. It also re-covers the markers fixed in #202.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.80%. Comparing base (f8b5407) to head (ad81444).

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #204   +/-   ##
=========================================
  Coverage     91.80%   91.80%           
  Complexity     3450     3450           
=========================================
  Files           104      104           
  Lines          9786     9786           
=========================================
  Hits           8984     8984           
  Misses          802      802           

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