Poisson Prove 0.2.1

Fixed

• With --json, stdout is the one JSON document whatever the target writes (#28). Before, a
  target that printed put its text in stdout beside the document and the sweep still exited 0, so
  a caller got success and a document it could not parse. Now everything the target writes to
  stdout goes to stderr instead, once, as it is written: a print in the rollout or at import, a
  write to sys.stdout.buffer, os.write(1, ...), the C library, a program the target starts,
  an atexit hook, and all of these under --episode-timeout. The same holds for
  ui --open --json, whose browser inherits stdout, and for what the Modal client or a Burla
  sweep's own process writes. The document, the exit codes and everything without --json are
  as they were.
• What that means for a caller of sweep --json: read both streams, because a caller that pipes
  stdout and stderr and reads only stdout can hang on a target that prints a lot (200 kB a
  rollout is enough; subprocess.run(capture_output=True) reads both); look for the line that
  starts error: , not stderr's first line, which can be the target's text; and with stderr a
  terminal the target sees sys.stdout.isatty() true, so a framework may draw there.
• A target that calls sys.exit, at import or in a rollout, no longer ends sweep --json with
  exit 0 and an empty stdout: the sweep exits 3 with one error: line. Without --json it ends
  the process as before.
• A target that closes its stdout no longer costs sweep --json its document.
• main() called in the caller's own process with --json and no stdout= leaves file
  descriptor 1 pointing at stderr for the rest of that process, so a host script that prints
  afterwards prints to stderr; Prove's own output from later calls still goes to the process's
  real stdout. A caller that passes stdout= or uses contextlib.redirect_stdout is untouched.
• Under --episode-timeout, text a rollout printed and had not yet flushed is no longer lost
  when the episode's process is ended. This holds without --json too, where it was lost in some
  runs.
• On Burla, a rollout that writes to stdout through the file descriptor (a flushed print,
  os.write, a child process, or any print under --episode-timeout) no longer fails the
  sweep with an UnpicklingError, with or without --json. What a rollout writes stays with the
  worker.
• The dashboard no longer cuts a chart off at its panel's edge (#31). On the comparison page the
  heatmaps' last column was cut at 1,280, 1,440 and 1,920 wide, and on a phone the differences
  and limits charts and the charts on a condition's page were cut too. A chart now leaves room
  for its own padding inside its panel; a chart in a panel wider than the chart is drawn as
  before.