Closes the BasicHTTP front example's gaps

[johnnyt]

Closes three gaps in the BasicHTTP front example (the library hold desk).

What changes

• The refused-desk test proves the hold ended unreached. It now
  asserts that error.communication, carrying the send id, was re-entered
  into that execution (the [:statifier_persistence, :execution, :step, :reentered] telemetry event), beside the existing completed status and
  the 404 at the location. The chart has two finals and only
  error.communication leads to desk_unreached; a finished execution
  keeps no configuration to read the final's id from, which is why the
  test reads the event rather than the state.
• The no-target case is named for what it is. Statifier 2.10.0 (the
  version mix.lock resolves) plans a BasicHTTP send with no target as a
  {:raise, :platform, "error.communication", ...} instruction carrying
  the send id, and makes no request
  (Statifier.Send.BasicHTTP.deliver/3). HoldDesk's executor used to
  reach that instruction through a catch-all named
  :basichttp_send_not_planned. It now matches the raise and fails the
  send as {:basichttp_send_without_target, send_id}. The halt is kept,
  and named truthfully, because it IS the planned outcome here: a durable
  execution has no internal queue to raise on, and statifier_persistence
  0.24.0 re-enters a failed send as error.communication with the same
  content origin and the same sendid the raise carries
  (StatifierPersistence.Executions, the error re-entry). Any other
  instruction still halts, now as :basichttp_instruction_unknown. A new
  test drives execute/2 with a no-target send and asserts the failure
  and that no POST was made.
• The guide and the controller moduledoc list the 500 answer.
  StatifierRouter.BasicHTTP.Front.response/1 answers 500 for any answer
  it does not otherwise map, a delivery that did not settle, which a
  sender may retry. The guide's status table and
  StatifierExamplesWeb.BasicHTTPController's moduledoc now say so; the
  guide and HoldDesk's moduledoc also say how a no-target send reaches
  the execution.

Checks

• Full mix quality green on the committed tree.
• Sabotage, one row per check, each restored byte-equal from a copy:
  the raise clause made to continue turned the no-target test red on
  its assertion; a refused POST swallowed in perform/3 turned the
  refused-desk test red on the re-entry assertion.

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