Fix: strip surplus indent on over-indented list-item paragraph continuation

[dereuromark]

Problem

A tight list-item paragraph continuation line indented more than the item's
content indent kept the surplus whitespace in the rendered inline text.

Input:

- a
  - b
    c

Before this fix djot-php rendered the continuation as c (two leading
spaces preserved):

<ul>
<li>
a
- b
  c
</li>
</ul>

Canonical djot.js (the djot/djot npm package) renders every indentation
variant of this case identically, with the continuation stripped to c. The
four variants (c flush, one-space, content-indent, over-indented) should all
produce the same output.

Root cause

In BlockParser, list-item continuation lines are collected with their indent
kept relative to the item content indent (stripLeadingIndent($nextLine, $contentIndent)).
That relative indentation is needed for the branch that parses an item's lines
as nested blocks (code fences, blockquotes, etc.).

The tight single-paragraph branch, however, joined those lines and handed them
straight to the inline parser. The inline parser emits a soft break for each
newline without trimming leading whitespace, so any surplus indent leaked into
the text. The dedicated paragraph collector (tryParseParagraph) already trims
leading whitespace on continuation lines per the djot soft-break rule; the list
path did not.

Fix

In the single-paragraph branch only, strip all leading whitespace on the
collected continuation lines before inline parsing, matching tryParseParagraph
and canonical djot.js. The block-parse branch is untouched, so indentation
remains significant where it matters.

Result

All four indentation variants now match canonical djot.js output, and the
official conformance suite stays green.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.89%. Comparing base (ae55a79) to head (26652cb).

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #239   +/-   ##
=========================================
  Coverage     91.89%   91.89%           
  Complexity     3527     3527           
=========================================
  Files           106      106           
  Lines          9970     9970           
=========================================
  Hits           9162     9162           
  Misses          808      808           

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