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/SessionSetupRunTests.swift

@@ -13,6 +13,22 @@ struct SessionSetupRunTests {
         #expect(h.backend.calls == [.sweep(keeping: []), .fix(.serviceStopped), .preflight, .fix(.imageMissing), .preflight])
     }
 
+    @Test func aWindowRunFetchesTheKernelBetweenTheServiceAndTheBuild() async {
+        let h = Harness()
+        h.backend.queuePreflight(.issue(.kernelMissing), .issue(.imageMissing))
+        #expect(await h.controller.runSetup(.serviceStopped, continuing: true) == .ready)
+        #expect(h.backend.calls == [.sweep(keeping: []), .fix(.serviceStopped), .preflight, .fix(.kernelMissing), .preflight,
+                                    .fix(.imageMissing), .preflight])
+    }
+
+    @Test func aKernelThatStaysMissingIsReportedOnce() async {
+        let h = Harness()
+        h.backend.queuePreflight(.issue(.kernelMissing))
+        #expect(await h.controller.runSetup(.kernelMissing, continuing: true)
+                == .failed(.kernelMissing, SessionController.stillBrokenMessage))
+        #expect(h.backend.calls.filter { $0 == .fix(.kernelMissing) }.count == 1)
+    }
+
     @Test func aWindowRunStartsFolded() async {
         let h = Harness()
         h.backend.hold(.fix)
@@ -79,6 +95,22 @@ struct SessionSetupRunTests {
         #expect(h.backend.calls == [.sweep(keeping: []), .preflight, .fix(.serviceStopped), .preflight])
     }
 
+    @Test func checkingNeverDownloadsTheKernel() async {
+        let h = Harness()
+        h.backend.queuePreflight(.issue(.serviceStopped), .issue(.kernelMissing))
+        #expect(await h.controller.checkSetup() == .needs(.kernelMissing))
+        #expect(h.state == .idle)
+        #expect(h.backend.calls == [.sweep(keeping: []), .preflight, .fix(.serviceStopped), .preflight])
+    }
+
+    @Test func setupErrorsReadAsSentences() {
+        let timedOut = SessionController.message(for: CLIRunner.Failure.timedOut(["system", "start"]))
+        #expect(timedOut == "The container runtime stopped responding. Try again.")
+        #expect(SessionController.message(for: CLIRunner.Failure.notInstalled) == "Apple's container runtime isn't installed.")
+        #expect(SessionController.message(for: CLIRunner.Failure.launchFailed("denied"))
+                == "Couldn't run the container runtime: denied")
+    }
+
     @Test func checkingOpensALinkACardWasHolding() async {
         let h = Harness()
         h.backend.queuePreflight(.issue(.imageMissing))