rpc: a `trace` verb, and --trace for pane spawn

Driving the trace viewer needed the same treatment as everything else: seeking by
hand through a few hundred keypresses to reach an interesting timestamp is not a
way to test it.

  trace {seek: 120}        absolute timestamp
  trace {seek: "!50"}      halfway through, like Tenet's timestamp shell
  trace {goto: "main"}     first execution of a name or 0xADDR
  trace {step: 20}         relative, negative goes back
  trace {step: 5, over: true}

Returns the usual state snapshot plus {idx, length, pc, changed} so a driver can
see where it landed and what that instruction wrote.

pane spawn --trace FILE passes it through to the launcher, so a trace pane is one
command.

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