Apply adjacent-inline separator on all inline buffers in HtmlToDjot

[dereuromark]

Follow-up to #205, which fixed adjacent same-delimiter inline runs but only on the processChildren() path.

Problem

The separator in #205 covered paragraphs, but two other code paths buffer inline output by hand and so still produced the malformed token:

<em>a</em><em>b</em>                       ->  _a__b_     ->  <em>a_</em>b_     (processBlock buffer)
<ul><li><em>a</em><em>b</em></li></ul>     ->  - _a__b_   ->  <em>a_</em>b_     (processList buffer)

(Paragraph-wrapped input was already correct after #205.)

Fix

Extract the inline join into a shared appendInline() helper that inserts the {} separator when two pieces would merge a same-delimiter run, and use it on all three concatenation paths: processChildren(), the processBlock() inline buffer, and the processList() list-item buffer. Table cells already delegate to those, so they are covered too.

The property test gains bare-top-level and list-item adjacency cases across em, strong, sub, sup and code, asserting (whitespace-normalized) that the two runs stay separate regardless of the surrounding block.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.82%. Comparing base (a2aa1bb) to head (360b1c5).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #206      +/-   ##
============================================
+ Coverage     91.79%   91.82%   +0.03%     
- Complexity     3457     3458       +1     
============================================
  Files           104      104              
  Lines          9798     9799       +1     
============================================
+ Hits           8994     8998       +4     
+ Misses          804      801       -3     

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