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]>

vminitd/Sources/VminitdCore/IOCloser.swift

@@ -16,6 +16,9 @@
 
 #if os(Linux)
 
+import ContainerizationOS
+import Foundation
+
 protocol IOCloser: Sendable {
     var fileDescriptor: Int32 { get }
 
@@ -34,4 +37,25 @@ struct UnownedIOCloser: IOCloser {
     func close() throws {}
 }
 
+/// [Nucleic vendored patch] Owns a `dup(2)` of another descriptor. Epoll keys registrations
+/// on the fd NUMBER, so two IOPairs sharing one underlying file (TerminalIO: the stdout
+/// relay reads the terminal fd, the stdin relay writes it) collide the moment the stdin
+/// side needs an EPOLLOUT backpressure registration on the number the stdout side already
+/// registered — which now fails fast (EEXIST) and killed the stdin relay. A dup shares the
+/// open file description but has its own number, so each relay registers independently; the
+/// description stays valid until every descriptor over it is closed.
+struct DupIOCloser: IOCloser {
+    let fileDescriptor: Int32
+
+    init(duplicating fd: Int32) throws {
+        let duplicate = dup(fd)
+        guard duplicate != -1 else { throw POSIXError.fromErrno() }
+        self.fileDescriptor = duplicate
+    }
+
+    func close() throws {
+        guard Foundation.close(fileDescriptor) == 0 else { throw POSIXError.fromErrno() }
+    }
+}
+
 #endif