#21369 perf(cmux-next): avoid duplicate older-history loads

Summary

• avoid issuing duplicate older-history requests on every VirtualTranscript commit while the reader remains near the top
• key each request to the session and first loaded row, reset when leaving the threshold or when history loading is disabled, and retry transient failures

Why

Streaming, resize, and scroll-follow commits can happen many times while the transcript is near the top. The existing no-dependency effect called onLoadOlder() for each one; the direct client deduplicated in-flight requests, but completed pages could still trigger another request before the next page was needed. The keyed guard preserves page-in behavior while removing that repeated work.

Validation

• bun run typecheck
• bun run lint
• bun test src/agent-session/acpmux (48 passed)
• React Doctor was attempted locally with the repository's --fail-on none command but exceeded the bounded 120s local window; fleet CI remains the source of the full React Doctor report.

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Summary by cubic

Prevents the transcript from issuing duplicate older-history requests when the reader stays near the top during streaming, resizing, or scroll-follow commits.

• Older-history loads are now keyed to the session and first loaded row, so only one request fires per page while the view remains in the threshold.
• The guard resets when the reader leaves the threshold, when history loading is disabled, or after a transient failure, so failures stay retryable without turning every render into a request.