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

[TheLarkInn]

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.

Scope

• 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.

Acceptance criteria

• 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.

DAG

Terminal node; later-major cleanup only.

Part of #5858.