Paint a body with context rows once, into its layer, and join it

joinBody replaces the second paint of a file body with compact tree-sitter
context rows: the body is painted once into its body layer (more rows than
the grid has) and the grid's body rows are copies of the layer's first
ones; the rail is the grid's alone, from its region. Which rows are context
rows is still decided by a text build before the paint, and a body without
them reuses that text and paints straight onto the grid: every other body
has no row the grid cannot show, so it gets no layer and no copy.

Each part now paints only its own rows. The tag paints the tag rows of
every pointer sweep that crosses them (the select sweep in the tag's
colours, the look-hover affordance over it, a middle or right sweep over
both in its button's tint), which the body used to paint over the tag on
the grid alone; the body paints only body rows, and a select sweep begun on
the tag marks the body rows it crossed before the text is laid down, as it
did. Grid and layers agree, the snapshot goldens are byte-identical, and a
test checks the joined grid rows and caret against a plain paint of the
same body with the tag on top and at the bottom. The GUI goldens gain a
terminal pane scene.

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