A dispatch error names the symbol the author typed - #728

Closes #710.

  >>> print("a", "b")
  ui.print: got 2 arguments, want at most 1

ui.print does not exist. ui is promoted, so its methods ARE the top-level globals and the
provider name is not a symbol any script can use -- an author who takes the error at its
word and writes ui.print(...) gets `undefined: ui`. The error sent them toward a second,
unrelated error, which is worse than a vague message.

Pre-existing, not introduced by #708: flow has done this since promotion existed.

The rule
  Where a call site exists, an error names what the author typed; where none exists, it
  names the action. The two cannot be one name because the action name is a node's
  identity field -- it is in the serialized document and the checksum -- so tying it to
  placement would rewrite every saved workflow when a provider's placement changed.

    print("a", "b")          script evaluation -- the call site is right here
    -> print: got 2 arguments, want at most 1

    plan.failed("db down")   runs later, possibly from a workflow loaded off disk
    -> flow.failed executed: db down          unchanged; there is no call site to name

Three sites, and each already knew its own surface
    starlarkbridge/go_receiver.go   script     consults Placement(): bare when promoted
    plan/provider.go                workflow   promoted   -> plan.<method>
    plan/adapter.go                 workflow   qualified  -> plan.<provider>.<method>

  Only the first consults placement. The other two ARE the promoted and qualified paths,
  so the site itself determines the shape.

  This is what #715 unblocked. Before it, placement was a bit tangled with dispatch and
  "which name would the author have typed" had no clean answer.
  flags.Placement() == op.PlacementPromoted is now a one-line question.

actionName was a misnomer, and it made the change look risky
  Both workflow sites named the variable actionName and passed it to dispatchBuiltinBody,
  which reads as though changing it would move the serialized action name. It does not:
  that function uses the string ONLY in error text, and a node's action name comes from
  unit.Action().Name() at marshal. A display string was wearing an identity's name.
  Renamed to callSiteName at both sites, with a comment saying which it is.

Step 1 classified every fixture before anything changed
  Four expectations name a provider-qualified symbol; exactly ONE moves.
  test_imm_ui_print_replaces_builtin is script evaluation. The three file.* expectations
  and the flow.* one are execution errors and stay exactly as they are. "Runs a graph" is
  the mechanical discriminator -- every execution fixture calls t.run(graph).

  The change then confirmed the classification rather than merely being consistent with
  it: those four fixtures passed untouched, because they never reach these sites.

Tests assert both directions
  A promoted provider reports the bare name AND a qualified one keeps its qualifier --
  together, because a fix that made every name bare would satisfy the first alone. And
  flow.choose, flow.gather, and flow.failed must still resolve through BuildAction: a
  saved workflow says flow.choose and has to load.

  test_imm_ui_print_replaces_builtin pinned `ui\.print: got 2 arguments` deliberately,
  with a comment saying the fix to #710 had to come back through it. It did.