Pin MappingsConstantNameDedup.groupKey javadoc to the current three ErrorChannel arms

MappingsConstantNameDedup.groupKey javadoc (and its "the two namespaces never collide" analysis) describes only two ErrorChannel arms, PayloadClass and LocalContext, but the switch in groupKey/renameChannel now handles a third arm, ErrorChannel.Mapped, which also groups by mappingsConstantName. The collision analysis predates the third arm and understates the arm set, so a reader trusts a two-namespace guarantee the code no longer provides. Rewriting the prose to cover three arms would be fresh unpinned prose (the hazard the audit avoids); the durable fix is to pin the arm set structurally, for example an exhaustive switch or a test that fails when an ErrorChannel arm carrying a mappingsConstantName is added without updating the dedup grouping.

Surfaced by the R483 javadoc drift audit.