The rich text document pipeline (`DocumentRoot`, `ParagraphNode`,
`HeaderNode`, and therefore `flame_markdown`) was the last place in
Flame that broke lines itself. Its layout builder split on ASCII spaces,
re-measured every tentative line with a fresh `TextPainter`, could not
wrap CJK, threw on `TextAlign.justify`, lost kerning and ligatures at
span boundaries, and needed glue heuristics for punctuation that were
patched again in #4060 after #3192 and #3939.

Since `InlineTextStyle` always renders through `TextPaint`, that
pipeline can hand the whole block to Flutter's paragraph engine without
losing any capability:

- `InlineTextNode.layoutBuilder` is replaced by `InlineSpan
toInlineSpan()`. `PlainTextNode` and `GroupTextNode` build a `TextSpan`
tree, the wrapper nodes delegate to their child.
- `TextBlockNode.format` lays out one `TextPainter` per block and
returns the new `ParagraphTextElement`, which exposes `lines` and
`lineMetrics`. Justify, CJK wrapping, hard line breaks, and shaping
across differently styled spans now work.
- `TextElement` gained a no-op `dispose()`, forwarded by `GroupElement`,
which `ParagraphTextElement` uses to release its painter.
- `TextBoxComponent` lays out a dedicated painter for wrapping and reads
lines through `ParagraphTextElement`. It previously called
`layout(maxWidth:)` on the painter shared through the `TextPaint` cache,
so the wrapped layout leaked into later `format` calls for the same
string. Per-line rendering, `align`, `growingBox` and the typewriter
effect are unchanged.

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Co-authored-by: Luan Nico <[email protected]>