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/__tests__/AddLaneModal.test.tsx

@@ -245,10 +245,11 @@ describe("AddLaneModal — picking an existing branch (issue #34)", () => {
     await user.type(repoField, "/Users/tester/projects/repo");
     await screen.findByLabelText("Branch to fork from");
     await user.type(screen.getByLabelText("Title"), "Auto named");
-    // The placeholder previews the name the server will pick.
+    // No `feat/…` guess: the prefix is the server's LANE_BRANCH_PREFIX, which
+    // the form cannot see, so the placeholder only says the field is optional.
     expect(screen.getByLabelText("New branch name")).toHaveAttribute(
       "placeholder",
-      "feat/auto-named"
+      "Automatic — named from the title"
     );
 
     await user.click(screen.getByRole("button", { name: "Add lane" }));