fix(lanes,git): harden the lane git surface and fix client races in the Changes panel

Server (git-diff.js, git-ops.js, worktree.js, routes/lanes.js):
- Every lane git call runs with GIT_LITERAL_PATHSPECS=1 and -z porcelain, so a
  file name is a file name, never a glob, and odd names parse correctly.
- A lane that is a subdirectory of its repo sees and acts on ONLY its own
  paths (reads, stage/unstage/discard/commit, and gitFacts.files).
- Any file named settings.local.json (the lane's router API key) is refused as
  403 ESECRET on disk (symlinks resolved) and at every committed rev.
- commit refuses with 400 ESTAGEDOUTSIDE while anything outside the lane is
  staged (checked with --no-renames, so a move into the lane is caught).
- push resolves its remote (upstream → only remote → origin), else 400
  ENOREMOTE; unstaging a rename unstages both halves.
- Git writes serialize on the repo's git dir, not the lane id, so two lanes on
  one index cannot collide on index.lock.
- git refusing an action (a numeric exit status) answers 409 EGIT with git's
  stderr instead of a 500 carrying a numeric code; one shared parseNumstat.

Client:
- Changes panel: no stale answer from a previous lane or an older quick-open
  query repaints the panel; discard disarms on lane switch; Enter on a file
  path opens it instead of folding; bounded tree expand-all.
- Large diffs (> 500 rows) render only the visible rows, with stable width
  and in-sync split columns; mirrored split scroll is clamped.
- Git bar: no "in sync" without counts, a clear no-remote message, branch list
  re-read after a switch, and no previous-lane facts after a lane switch.
- Tooltips no longer flicker or close on unrelated scrolls.

client/src/components/lanes/LaneCard.tsx

@@ -55,6 +55,13 @@ function useLaneGitFacts(laneId: number): [LaneGitFacts | null, () => void] {
   // Bumped by the git action bar: a commit or a pull changes every fact on
   // this row, and waiting out the 30s poll makes the button look inert.
   const [nonce, setNonce] = useState(0);
+  // A lane switch drops the previous lane's facts during render, so nothing
+  // (the git bar's "current branch" included) shows them until the new read.
+  const [factsLane, setFactsLane] = useState(laneId);
+  if (factsLane !== laneId) {
+    setFactsLane(laneId);
+    setFacts(null);
+  }
 
   useEffect(() => {
     let alive = true;
@@ -854,7 +861,7 @@ export default function LaneCard({
                   <button
                     type="button"
                     onClick={() => onSelectLane?.(w.id)}
-                    className="truncate text-left text-blue-400 hover:underline"
+                    className="truncate text-left text-blue-700 dark:text-blue-400 hover:underline"
                     title={w.cwd}
                   >
                     #{w.id} {w.title || w.cwd} · {w.status}