Deep-sweep fixes: cooperative-pool starvation, stdio tail loss, epoll integrity, UI pins

Host — the two remaining app-wide stall mechanisms plus main-thread pins
found by mining all nine hang reports:
- LinuxProcess.startStdinRelay wrote to a BLOCKING stdin fd on a
  width-limited cooperative-pool thread, non-cancellably; wedged guests
  starved the whole concurrency runtime (decode loops, watchdogs — an
  app-wide freeze surviving the reconcile fix). Writes now offload to a
  per-process GCD queue (vendored patch #18).
- TranscriptWriter (actor) did blocking write/fsync on the cooperative
  pool; it now runs on its own DispatchSerialQueue executor.
- UserMessageBubble's truncation probe typeset entire pasted-log-sized
  messages through CoreText per layout pass (100% main-thread pins in
  the 07-21 hang reports); certainly-long messages now skip the probe
  and render a prefix while collapsed.
- toolGroupSignature JSON-encoded every tool input in the transcript up
  to 12.5x/s on the MainActor; now a structural hash. The summary pass
  is trailing-throttled to 0.4s, and flatItems joins streaming chunks
  once instead of re-copying the prefix per delta.
- StatusFeedFetcher.parseDate allocated three formatters per call (86%
  of a pool thread in the 07-26 report); now shared statics.

Guest (vminitd) — teardown data loss and epoll registration hazards:
- IOPair no longer closes on a bare EPOLLHUP with a backpressure flush
  in flight (dropped the CLI's final output line); EPOLLOUT finishes the
  flush, then EOF closes loss-free. ManagedProcess.setExit closes only
  stdin, letting stdout/stderr self-close on EOF, with an 8s grace pass
  (patch #16).
- Epoll events carry a registration generation; the supervisor ignores
  stale events for recycled fd numbers. registerFd refuses EEXIST
  instead of clobbering the existing handler. TerminalIO's stdin relay
  writes a dup of the terminal fd so its backpressure registration
  can't collide with the stdout relay's (patch #17).
- VsockProxy flushes bytes parked toward the surviving peer on hangup,
  closes the dialing socket on a failed backend connect, and
  StandardIO/TerminalIO clean up partially-created pairs on setup
  failure (patch #16).

Full suite: 1451+292+74+20 tests, two failures — both pre-existing
environmental (MacVM base image absent on this machine; a load-flaky
liveness test that passes 3/3 in isolation).

Co-Authored-By: Claude Fable 5 <[email protected]>

Sources/Containerization/LinuxProcess.swift

@@ -270,11 +270,33 @@ extension LinuxProcess {
     func startStdinRelay(handle: FileHandle) {
         guard let stdin = self.ioSetup.stdin else { return }
 
+        // [Nucleic vendored patch] The stdin fd is BLOCKING (only the read fds get
+        // O_NONBLOCK), and `FileHandle.write` parks its thread for as long as the guest
+        // isn't reading once the vsock buffer fills — with a large prompt line that can be
+        // forever, and the old direct call parked a *width-limited Swift cooperative-pool
+        // thread*, non-cancellably. A few wedged sessions starved the entire concurrency
+        // runtime: every exec's decode loop, first-output watchdogs, all of it — an
+        // app-wide stall. Offload each write to a per-process GCD queue so the pool thread
+        // suspends instead; a wedged write now costs one expendable GCD thread, and
+        // process deletion closing the fd still unwedges it.
+        let writeQueue = DispatchQueue(label: "com.nucleic.stdin-relay")
+        // The handle is used from one queue at a time (writes are serialized on writeQueue;
+        // the close paths run after the relay ends or via _closeStdin's own lock).
+        nonisolated(unsafe) let handle = handle
         self.state.withLock {
             $0.stdinRelay = Task {
                 for await data in stdin.reader.stream() {
                     do {
-                        try handle.write(contentsOf: data)
+                        try await withCheckedThrowingContinuation { (c: CheckedContinuation<Void, Error>) in
+                            writeQueue.async {
+                                do {
+                                    try handle.write(contentsOf: data)
+                                    c.resume()
+                                } catch {
+                                    c.resume(throwing: error)
+                                }
+                            }
+                        }
                     } catch {
                         self.logger?.error("failed to write to stdin: \(error)")
                         break