Fix the follow-up review's findings

- Pairing saves nothing if it was cancelled while adding the key.
- The ssh stand-in runs under bash: dash refuses job control without a
  terminal, which cost stdin and the process group.
- A server that stops while the tunnel opens fails the attempt (and retries)
  instead of leaving it half-connected.
- The health probe answers within its deadline even when a dead link keeps
  the probe itself waiting.
- A cached version is deleted only if no server runs from it (servers now run
  by absolute path) and it's two weeks unused.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

ios/FrameControl/SSH/FrameLink.swift

@@ -80,15 +80,15 @@ final class FrameLink: @unchecked Sendable {
 
     var isConnected: Bool { client.isConnected }
 
-    /// Whether the Frame answers a trivial command within a few seconds.
+    /// Whether the Frame answers a trivial command within a few seconds. The probe
+    /// runs unstructured: a dead link can keep it waiting well past the deadline,
+    /// and the answer mustn't wait for it.
     func answers(within seconds: Double = 6) async -> Bool {
         guard client.isConnected else { return false }
-        return await withTaskGroup(of: Bool.self) { group in
-            group.addTask { (try? await self.run("true").status) == 0 }
-            group.addTask { try? await Task.sleep(nanoseconds: UInt64(seconds * 1e9)); return false }
-            let first = await group.next() ?? false
-            group.cancelAll()
-            return first
+        let once = Once()
+        return await withCheckedContinuation { (c: CheckedContinuation<Bool, Never>) in
+            Task { let ok = (try? await self.run("true").status) == 0; if once.claim() { c.resume(returning: ok) } }
+            Task { try? await Task.sleep(nanoseconds: UInt64(seconds * 1e9)); if once.claim() { c.resume(returning: false) } }
         }
     }
 
@@ -133,6 +133,13 @@ final class FrameLink: @unchecked Sendable {
     }
 }
 
+/// True for the first caller only.
+final class Once: @unchecked Sendable {
+    private let lock = NSLock()
+    private var done = false
+    func claim() -> Bool { lock.withLock { defer { done = true }; return !done } }
+}
+
 func shellQuote(_ s: String) -> String {
     "'" + s.replacingOccurrences(of: "'", with: "'\\''") + "'"
 }

ios/FrameControl/SSH/HeadsetServer.swift

@@ -69,18 +69,20 @@ final class HeadsetServer: @unchecked Sendable {
             try await link.check("rm -rf \(dir).tmp && mkdir \(dir).tmp && tar xzf \(archive) -C \(dir).tmp && rm -f \(archive) "
                                  + "&& rm -rf \(dir) && mv \(dir).tmp \(dir)", "Couldn't unpack Frame Control on the headset")
         }
-        // Another phone or iPad may be running a different version right now, so only
-        // versions (and interrupted unpacks) untouched for two weeks go. This one is
-        // marked as used.
-        _ = try? await link.run("touch \(dir) && find \(cacheDir) -mindepth 1 -maxdepth 1 -type d ! -name \(bundle.version) "
-                                + "-mtime +14 -exec rm -rf {} +")
+        // Another phone or iPad may be running a different version right now: a version
+        // goes only when no server runs from it and it hasn't been used for two weeks
+        // (this one is marked as used). Servers run by absolute path, so pgrep sees it.
+        _ = try? await link.run("touch \(dir) && cd \(cacheDir) && for d in */; do d=${d%/}; "
+                                + "[ \"$d\" = \(bundle.version) ] && continue; "
+                                + "[ -n \"$(find \"$d\" -maxdepth 0 -mtime +14)\" ] || continue; "
+                                + "pgrep -f \"$PWD/$d/\" >/dev/null && continue; rm -rf -- \"$d\"; done")
         return dir
     }
 
     /// Starts the server in dir and waits for it to say which port it took.
     static func start(in dir: String, over link: FrameLink, key: String, device: String) async throws -> HeadsetServer {
         let command = "cd \(dir) && FRAME_LOCAL=1 FRAME_UI_KEY=\(key) FRAME_DEVICE=\(shellQuote(device)) "
-            + "exec python3 -I -u -B ui/server.py --port 0 --exit-on-eof 2>&1"
+            + "exec python3 -I -u -B \"$PWD/ui/server.py\" --port 0 --exit-on-eof 2>&1"
         let stream = try await link.client.executeCommandStream(command)
         let box = PortWaiter()
         let reader = Task { () -> Void in