A 9P write scrolls only the view: dot moves as acme's textinsert moves it, never to the end of the write

writeData, flushBatch and writeBody showed what they wrote with
showOffset, which put the cursor at the write's end. The cursor is dot's
head, so a data write or a body append moved dot there, and a `sel` write
after it replaced the wrong text. showOffset now scrolls the view to the
offset and leaves the cursor, pinned, where it was. Dot shifts only by
the rule it already followed, acme's: an insert before dot shifts it, one
at its start or inside it grows it, one at its end or after it leaves it.
The test covers each case, then a body append followed by a sel write.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

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