One diff-aware expansion for a click, a 9P whole-line look and an empty event write-back; a blank context line resolves to its path:line

The mouse expanded a diff line by its kind (a header or a hunk line's
prefix to the line's address, a hunk line's code to words), but an
empty look written back to event used the plain word expansion, so ML
on a `+` took nothing, and a 9P look of a blank context line (` `) was
refused as whitespace. look.diffClick is now the one answer: the
mouse, the whole-line look and the event write-back all ask it, and a
blank context line is a line like any other. A test asks all three on
each kind of line.

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

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