The replay's two legacy handlers are uncalled: addHistoryMessage / prependHistoryMessage and the kind: "history" branch

Filed by cycle cyc20260921-051840 when it closed #1472 (the acceptance of PR #1489 re-measured
on master), so the residual #1489 stated in its body is not lost with that issue.

This is the same class as the "second mapping" rant 2026-09-20T18:58:44 forbids, in its dying
form: the handlers that were the second record→entry mapping are now dead code, kept alive only by
their own declarations and a comment naming their status.

Measured on master bff15afc

grep -rn "addHistoryMessage|prependHistoryMessage" --include=*.ts --include=*.tsx emrg/gui/renderer/src
  transcript.ts:163-169   the doc comment + the two interface members ("已无调用方" / no call site)
  transcript.ts:502       addHistoryMessage: (text, sid, role) => { … entries.push({ kind: "history", … }) }
  transcript.ts:518       prependHistoryMessage: … entries.unshift({ kind: "history", … })

No production call site exists outside transcript.ts itself, and nothing in the test suite calls
either (the Shell path now goes through lib/historyReplay.ts). kind: "history" is written only
from those two implementations (:514, :531).

Why it is not just tidiness

Every remaining member is a second way to put history into the transcript, which is exactly what the
rant's acceptance forbids — and it is reachable: the entry kind is rendered, so a future change that
calls either handler silently produces history that did not come through the same handlers as the
live stream. Dead code that is one call away from being a second mapping is the failure mode the
rant named, so it should not be left as a permanent option.

What would close it

Delete the two interface members, their two implementations, the kind: "history" render branch and
its CSS, and any test that constructs them directly — one change, no behaviour change, with the
renderer suite green afterwards (Agent.md's Node count line needs re-syncing via
scripts/check-node-test-count.py --write if a case is removed with it).