Layout: only Paragraph supports inline runs — Heading and ListItem take one string

[Tim81]

Paragraph supports mixed inline styling — it takes IEnumerable<TextRun> and has
Add(string, TextStyle?). No other text-bearing element does.

Element	Constructor	Content
Paragraph	Paragraph(IEnumerable<TextRun>), Add(string, TextStyle?)	runs
Heading	Heading(string text, TextStyle? style)	one string, one style
ListItem	ListItem(string text, TextStyle? style)	one string, one style
Cell	Cell(string content)	one string — see L0c

ListItem.AddChild adds nested list items, not inline content, so it does not help.

What that makes impossible

Ordinary things, not exotic ones:

• bolding or italicising a word in a heading — "The Save method", "Ceci and cela"
• a link inside a list item — arguably the single most common thing a list item contains
• a superscript, a subscript, or a footnote reference (L12) anywhere but a paragraph
• a differently-languaged run inside a heading, which PDF/UA-2 §8.4.4 requires be declarable at the
  point the language changes — currently only Heading.Language exists, applying to the whole heading

That last one is a conformance consequence, not just a convenience one.

Scope

Heading and ListItem take runs, exactly as Paragraph does. The single-string constructors stay —
they are Shipped, and they are the common case — as conveniences over the general form.

This is the same content-model problem as L0c, and the three should be solved together with one shape
rather than three similar ones.

Acceptance

• A heading with a bolded word and an inline code span.
• A list item containing a link and a footnote reference.
• A run in a different language inside a heading, emitting /Lang on the right structure element.
• Existing single-string constructors produce byte-identical output.
• Structure tree correct: inline runs inside a heading are content items of one Hn, not separate
  elements — the same principle L0a establishes for fragments.

Relates to

• L0c — table cells, the third member of this family.
• L0a — one structure element per logical element.
• L12, L26, L28 — footnote references, decoration and links, all of which need somewhere inline to live.

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Plan labels used above, resolved: L0a #285 · L0c #287 · L12 #297 · L28 #299 · L26 #305.