feat(lanes): stage/undo one hunk at a time, sticky review file headers

Review and the single-file view gain per-hunk Stage and Undo
(stage-hunk / discard-hunk: the hunk is named by its @@ line and
re-matched against a fresh diff, then git apply --cached / -R; undo
needs confirm: true and two clicks). Each review file header stays
pinned while its diff scrolls and carries the file's own
stage/unstage/discard.

docs/LANES.md

@@ -601,6 +601,11 @@ only one that answers "show me the code" rather than "show me the change"):
   target-practice exercise against one chevron.) One toolbar button folds or
   unfolds all of them. What is remembered is the set of *collapsed* paths, so
   a file that appears later is open by default; a lane switch clears it.
+  Each file's header stays **pinned** at the top while its diff scrolls past,
+  and carries the same stage / unstage / discard buttons as its overview row.
+  On a file with unstaged changes every `@@` row adds **Stage** and **Undo**
+  for that hunk alone (`stage-hunk` / `discard-hunk`); Undo asks twice, like
+  discard. The single-file view offers the same hunk buttons.
 - **One file** — clicking a file's path in any list opens just that file.
   **All files** returns to the list it was opened from — the same mode, at the
   same scroll offset — because reading a change-set is a walk down a long list