[rush reporter][R11] Remove legacy reporter bridges in a later major

Parent: #5858

Blocked by: #5982, #5983, plus at least one full major of default use with no blocking canary regressions

Goal

Track the intentionally deferred cleanup after the reporter system has proven ecosystem adoption.

Define measurable ecosystem/canary criteria for removing the legacy renderer.
Remove the AlreadyReportedError bridge only after migration criteria are met.
Remove obsolete primary-path StreamCollator plumbing while retaining the standalone package if other consumers still need it.
Remove temporary compatibility adapters and update protocol support policy as appropriate.

At least one full major has shipped with the new reporter defaults.
Canary and compatibility telemetry show no blocking regressions.
Migration documentation and release notes provide replacement guidance.
Cleanup does not remove permanent verbosity aliases or required emergency/recovery behavior without a separate approved design.

Terminal node; later-major cleanup only.

Part of #5858.