electron+tests: zoom plumbing (construction-only lights), hermeticity fence for the jobs suite

- ZOOM: the ⌘±/⌘0 ladder is owned, not stock roles — applyZoom sets the
  factor and broadcasts 'lh:zoom'; the renderer publishes --lh-zoom; the
  gutter lengths (.lh-lights-gutter / .lh-closed-toggle) divide by it so
  their RENDERED size is a physical constant at every zoom — the graph
  button stays glued next to the lights, nothing slides under them.
  before-input-event catches ⌘=/layout variants the menu's Plus may not
  claim; did-finish-load re-pushes on reload; getZoom covers first paint.
  The lights themselves stay at construction position — Electron 44 has
  NO setTrafficLightPosition (options-only; the first run crashed on
  it). HIG-acceptable: native lights hug the corner; the web chrome
  compensates around them.
- JOBS SUITE FLAKE, actually fixed this time: the seams are
  module-global and the sweep scans every record — a timed-out async
  test left a running orphan + late wake that poisoned its NEIGHBORS
  (respawned 3 !== 0). New afterEach fence: settle every tracked
  running job, unregister aborts, seams reset to inert no-ops — every
  test starts from identical state regardless of upstream flake.
  Verified: two consecutive full-suite runs 1326/1326.

electron/preload.cjs

@@ -8,10 +8,22 @@
  *   threshold (928px) when the sidebar is opened below it. The main
  *   process clamps: no-op if the window is already wide enough.
  *
+ *   window.lh.onZoom(cb) / window.lh.getZoom() — the zoom truth (owner
+ *   round 7, 2026-10-05): every ⌘±/⌘0 change arrives as "lh:zoom" with
+ *   the new factor; getZoom covers the first-paint race. The renderer
+ *   publishes it as --lh-zoom so physical-constant lengths (the traffic-
+ *   light gutter) can divide by it.
+ *
  * Channel names are mirrored in electron/main.cjs — keep them in sync.
  */
 const { contextBridge, ipcRenderer } = require("electron");
 
 contextBridge.exposeInMainWorld("lh", {
 	expandWindow: () => ipcRenderer.send("lh:expand-window"),
+	onZoom: (cb) => {
+		const listener = (_event, factor) => cb(factor);
+		ipcRenderer.on("lh:zoom", listener);
+		return () => ipcRenderer.removeListener("lh:zoom", listener);
+	},
+	getZoom: () => ipcRenderer.invoke("lh:zoom:get"),
 });