The last artifacts carrying retired placement vocabulary - #729

Closes #725.

#715 renamed the types, constants, accessors, directives, and test functions. Two
artifacts still spelled the retired word.

  pkg/op/provider/ui/root_test.go -> promotion_test.go
    Its tests became TestUIPromoted_* and the filename did not follow. Named for the
    OUTCOME rather than the axis: both tests are about a promoted provider, and a
    qualified provider's tests would sensibly live elsewhere. git mv, so history follows.

  tierCollisionRootProvider -> tierCollisionPromotedProvider
  collisionRootReceiverType -> collisionPromotedReceiverType
    A synthetic fixture declaring PlacementPromoted, described as a "root provider".

What was deliberately NOT renamed, and why it matters
  These contain "root" in a FILESYSTEM sense, and a word-level sweep would break their
  meaning without breaking the build -- which is the failure mode #716's name-mapping
  appendix exists to prevent, applied here:

    pkg/fsroot/**                                 51 occurrences
    plan/root_binding_test.go                     the RUN root
    TestRun_BindsPendingResourcesToTheRunRoot
    TestGlob_RefusesPatternsOutsideTheRoot        the scoped root
    TestNewRuntimeEnvironment_MintsRootFromAnchor
    TestLintCommands_LoadConfigFromGitRoot        the git root

  Verified untouched by their absence from this diff, which is the only way a table of
  changes can distinguish "correctly unchanged" from "missed".

One correction to the issue's acceptance
  It asks that "the two historical notes survive". Only one still NAMES the retired
  identifiers -- receiver_type.go, explaining why ProviderRole, RoleAction, and
  Dispatch() went stale. The receiver_registry.go note survives as a note but was
  rewritten in current vocabulary during #715, so it records the doc correction without
  naming dead symbols. Better, but the acceptance implied two and there is one.