Simplify process forwarding and keep shim Notices in order

[SaladDay]

B4 simplicity audit, lane L3: process forwarding and the view. Net −117 production lines and +17 test lines, in 8 commits.

Defects fixed:

• Notice order (shim IPC step 8). A Notice is now an item on fd 2's output queue, so it follows the stderr already queued. When End stops the pump, the pump still writes the queued Notices as far as fd 2 takes them, so teardown stays bounded. Tests: TestNoticeFollowsQueuedStderr and TestNoticeOutlivesEnd, which covers stderr both writable and full.
• Teardown log levels. A normal view end and a Cancel log nothing above DEBUG. A real relay loss still logs ERROR and fails the Session.
  • The relay ignores the view's signals, so the launcher reaps it only when the relay ended first. The launcher reports that, View.RelayLost closes before Wait returns, and the agent host reacts to it. This replaces a Signal(0) probe that raced the Harness's exit.
  • A broker that stops serving a live relay now ends the relay's connection.

Deletions:

• processbroker:
  • the 410-line fake process service, replaced by a scripted single-operation peer for the four fault tests; their assertions are kept, and they no longer start processserve;
  • the busyAck transport, which rewrote an applied acknowledgement into Busy;
  • broker_other.go;
  • the doc's restatement of the relay.
• sessionview:
  • the non-Linux stubs except Init;
  • Process.Groups, keeping an explicit empty group list, which is now asserted;
  • Exit.CoreDumped;
  • one Stdio helper for the view's process, Spawn and the agent host.
• processshim:
  • the variadic stop flags;
  • frameBuffer, replaced by bytes.Buffer;
  • Conn.Unix.

The pipes the helper creates are now owned by the Session uid, as the view's own pipes already were. Everything in the view runs as that uid, so trust is unchanged; blind review accepted this.

Testing:

• The sessionview, processbroker, processshim and agenthost suites in privileged containers, on all CPUs and with --cpuset-cpus=0.
• The new and changed tests at -count=50, plus -race where the view allows it.
• Mutation checks on both defect fixes.
• Blind review (Codex): its three findings are fixed in f5cf15a.

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