test(e2e): give the job specs their own state back

Both specs stage a job through `/__test/emit` and neither cleared it.
Nothing resets those stores, so the spec that staged it is the one that
should put it back, and `JobsChanged` with `[]` is the whole cleanup --
`JobStore` replaces its list from every snapshot, so `testctl` needs no
special case.

**The leak as reported did not reproduce, and that is worth recording
rather than quietly fixing.**  Measured with a temporary probe: a
positive control confirmed a staged job really does move the shell at a
phone width (`job-band` renders a row, `.main-panel`'s top goes 0 to
55), and the very next page had no job at all.  The reason is that
every test gets a fresh page and `JobStore.init()` refetches `GetJobs()`
from a backend registry `/__test/emit` never writes to -- it calls
`events.Deliver`, which touches frontends and no state.  So the state
cannot cross a spec boundary as described, and the 55px offset the
draft assertion saw in that suite run has another cause that is not in
evidence.

The cleanup stays, because it costs a line and the leak would need only
one spec that keeps a page alive, and the comments say what was
measured rather than asserting the mechanism.

The durable half is the rule, now in the harness reference: measure
against the element next to you, not an absolute coordinate.  An
absolute number in a shell measurement is also a claim about everything
above it -- `contentTop === 0` asserts "and no background job is
running", which that spec could not arrange.

Closes #168

e2e/specs/jobs-on-a-phone.spec.ts

@@ -60,6 +60,23 @@ const PHONE = { width: 424, height: 439 };
 const DESKTOP = { width: 1100, height: 800 };
 
 test.describe('background jobs on a phone', () => {
+  /**
+   * **State a spec stages is the spec's to clear.** `/__test/emit` writes
+   * to a store nothing resets, so the event that staged a job is the
+   * event that clears it — `JobStore` replaces its whole list from every
+   * snapshot, so `testctl` needs no special case.
+   *
+   * **Measured on #168: this does not currently outlive the page.** Every
+   * test gets a fresh page, and `JobStore.init()` refetches `GetJobs()`
+   * from a backend registry that `/__test/emit` never writes to, so the
+   * staged job is gone before the next spec starts. Ownership is stated
+   * rather than a live leak repaired — the leak needs a page that
+   * survives its own spec, and there is none today.
+   */
+  test.afterEach(async ({ testctl }) => {
+    await testctl.emit('JobsChanged', []);
+  });
+
   test('are shown in the band, without opening anything', async ({
     app,
     testctl,