fix(custom-comments): heavy-box interiors are valid comments

The heavy-box empty and text lines began with a bare decoration char, so in line-comment languages (elisp, Python) C-; C h injected syntax-breaking lines. Prefix the interiors with the comment char and suffix them like cj/--comment-box does. Add the missing min-length guard so small or negative widths error cleanly instead of failing inside make-string. Updated the two characterization assertions to the corrected output.

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