A diff line looks up the path and line it names, as any look does

The look on a diff line was its own resolver: a walk up to .git/.jj from
the diff's directory, its own "no such file here" and "deleted by this
diff". A review tool's cached patch, in no repository, missed every file
though the session's terminals sat in the checkout.

Now a diff line only expands and transforms. Its expansion (what the hover
shows) is the whole line for a header, a `@@` line, or a hunk line
pointed at on its prefix, and its text is the look address the line names
(look.diffAddress): `path` for `diff --git`/`---`/`+++`, `path:c` for
`@@ -a,b +c,d @@`, `path:N` for a hunk line's new line (a removed line's,
the new line now where it was). That text goes through the regular look
exactly as if selected by hand: same resolution, placement and errors. A
9P look of a whole diff line takes the same transform.

Kept: the line classification, hunk arithmetic, git prefix stripping and
timestamp cutting (diff.zig). Gone: the repo-root walk, its messages,
Target.old_side and deletesAhead, and the click's row stash
(Pardes.diff_look). A test compares a B3 on a cached patch's line with a
look of the hand-selected `entry/src/entry.rs:2` in the same session.

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

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