Recognize an indented list marker at a block boundary

[dereuromark]

What

djot treats indentation as significant only for nesting (doc/syntax.md: "Indentation is only significant for list item or footnote nesting"). So a leading-indented list marker at the top level, or after a blank line, is simply a list. Reference djot parses these as lists:

input	reference	djot-php (before)
- item	<ul><li>item	<p>- item</p>
\t- item	<ul><li>item	<p>- item</p>
1. item	<ol><li>item	<p>1. item</p>

djot-php matched the marker against the raw line, so leading whitespace made parseListItemMarker fail and the line became a paragraph.

Fix

Match the marker on the de-indented line in tryParseList(). The leading indent still becomes the list's base indentation (nested lists already run with a non-zero base, so the existing machinery handles it).

Lazy-continuation lines are unaffected, because they never reach tryParseList():

• an indented marker continuing an open paragraph (text / - item, no blank) is collected by the paragraph;
• an indented marker after a list item without a blank line (- a / - b) is collected as that item's continuation.

Both verified to still render as text.

Scope / known edge

This is the parser-side tab gap B from #230/#231. Covers the common case (uniform leading indent). One rare divergence remains: a first item indented with a less-indented sibling ( - a / - b) ends the list early in djot-php, where reference keeps one list. Vanishingly rare; left for the tab-stop work (gap C) if it matters.

Verification

Full unit suite + official djot corpus pass (2498 tests).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.89%. Comparing base (52a66ea) to head (e41c25c).

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #232   +/-   ##
=========================================
  Coverage     91.89%   91.89%           
  Complexity     3523     3523           
=========================================
  Files           106      106           
  Lines          9958     9958           
=========================================
  Hits           9151     9151           
  Misses          807      807           

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