Fix first-run setup timing out on the kernel download

On a Mac without the Linux kernel, setup ran `container system start
--enable-kernel-install` with a 120 s limit. That command downloads a
697 MB archive without printing anything, so most connections hit the
limit and the window showed `timedOut([...])`.

- Start the service with --disable-kernel-install (10 min: a first
  start still pulls the 69 MB base filesystem).
- Download the kernel in its own step with progress, then hand the file
  to `container system kernel set --tar ... --digest ...`, which checks
  it against the runtime's digest.
- Opening the setup window no longer starts the download; Set Up does.
- Timeouts and runner errors read as sentences.
- A rebuild deletes the image it replaced, which frees about 2 GB each
  time, and deletes or stops the builder VM unless it was already
  running (it kept about 2.8 GB of memory).
- Setup asks for 10 GB free, measured on a real setup.

Tests/KnotchyCoreTests/SessionSetupTests.swift

@@ -270,6 +270,15 @@ struct SessionSetupTests {
                                     .start("knotchy-00000002")])
     }
 
+    @Test func aLinkFetchesAMissingKernelAfterStartingTheService() async {
+        let h = Harness()
+        h.backend.queuePreflight(.issue(.serviceStopped), .issue(.kernelMissing))
+        h.controller.open(Harness.url)
+        await waitUntil("loading") { h.state == .starting(.loading, folded: false) }
+        #expect(h.backend.calls == [.sweep(keeping: []), .preflight, .fix(.serviceStopped), .preflight,
+                                    .fix(.kernelMissing), .preflight, .start("knotchy-00000003")])
+    }
+
     @Test func aServiceThatWontStayUpShowsItsCardOnce() async {
         let h = Harness()
         h.backend.queuePreflight(.issue(.serviceStopped), .issue(.serviceStopped))