Separate adjacent same-delimiter inline runs in HtmlToDjot + doc fixes

[dereuromark]

Follow-up to the HtmlToDjot round-trip work (#202, #203, #204).

Problem (G2)

Two adjacent inline elements that share a Djot delimiter merged into a single malformed token on the round-trip:

<em>a</em><em>b</em>                  ->  _a__b_   ->  <em>a_</em>b_
<strong>a</strong><strong>b</strong>  ->  *a**b*   ->  <strong>a*</strong>b*
<code>a</code><code>b</code>          ->  `a``b`   ->  <code>a``b</code>

Same for <sub> and <sup>. Brace-delimited inlines (<del>, <mark>, <ins>) and mixed delimiters (<em> then <strong>) were already fine.

Fix

When concatenating inline children, insert an empty attribute group {} between two pieces where the left ends and the right begins with the same delimiter (_, *, ~, ^, or backtick). {} renders to nothing, so the content is unchanged while the two runs stay distinct:

<em>a</em><em>b</em>  ->  _a_{}_b_  ->  <em>a</em><em>b</em>

A trailing escaped delimiter (\_, produced for literal text) is recognized and left alone, so genuine literal characters are not affected.

The round-trip property test gains an adjacency sweep over em, strong, sub, sup, code, del, mark, ins (pairs and triples), plus the mixed-delimiter and space-separated cases as regression guards.

Docs (G3)

Fixes two overclaims in the converter guide:

• Round-trip mode is described as "lossless for supported constructs" instead of "perfect" (HTML is many-to-one with Djot, so perfect round-trips of arbitrary input are not possible).
• Code blocks preserve language and content; the fence length is normalized to the shortest safe length, not preserved. The table entry now says so.

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.79%. Comparing base (637623e) to head (f9363cb).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Converter/HtmlToDjot.php	84.61%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #205      +/-   ##
============================================
- Coverage     91.80%   91.79%   -0.02%     
- Complexity     3450     3457       +7     
============================================
  Files           104      104              
  Lines          9786     9798      +12     
============================================
+ Hits           8984     8994      +10     
- Misses          802      804       +2     

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