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/IOPair.swift

@@ -180,7 +180,16 @@ final class IOPair: Sendable {
 
                 if mask.isHangup && !mask.readyToRead {
                     self.logger?.debug("received EPOLLHUP with no EPOLLIN")
-                    if !ignoreHup {
+                    // [Nucleic vendored patch] Never close on a bare HUP while a backpressure
+                    // flush is in flight: the writer closing right after its final burst was
+                    // stashed in `pending` (destination momentarily full) used to hit this
+                    // close — whose single best-effort flush pass EAGAINed — and DROP the tail
+                    // of the stream (the CLI's final result line). With pending outstanding,
+                    // just return: the destination's EPOLLOUT edge flushes the backlog, the
+                    // pump then reads the drained closed-writer pipe, observes EOF, and closes
+                    // loss-free. If the destination dies instead, its own error/EPOLLERR wakes
+                    // the write handler, whose failed flush closes the pair — no orphan.
+                    if !ignoreHup && io.pending.isEmpty && !io.writeFdRegistered {
                         io.close(logger: self.logger)
                     }
                     return