Coalesce terminal replay batches server-side

[danshapiro]

Summary

• Coalesce contiguous ReplayRing replay batches before TerminalStreamBroker sends terminal.output payloads
• Preserve existing seqStart/seqEnd, attachRequestId, replay gap, and byte-budget behavior
• Remove the client replay-specific drain budget so replay mode only coalesces adjacent queued writes and does not change live/replay fairness

Investigation

• Current merged main sends 1,000 small live frames as 1 terminal.output payload, but 1,000 small replay frames as 1,000 terminal.output payloads
• This branch changes the same 1,000-frame replay probe to 1 payload spanning seq 1-1000, within MAX_REALTIME_MESSAGE_BYTES

Verification

• Red test before fix: coalesces contiguous replay frames before sending terminal.output payloads failed with 1,000 replay outputs instead of 1
• timeout 120s npm run test:vitest -- --run test/unit/client/components/terminal/terminal-write-queue.test.ts
• timeout 180s npm run test:vitest -- --run test/e2e/terminal-create-attach-ordering.test.tsx test/unit/client/components/TerminalView.lifecycle.test.tsx test/unit/client/components/terminal/terminal-write-queue.test.ts
• timeout 240s npm run test:vitest -- --run test/unit/server/terminal-stream/replay-ring.test.ts test/unit/server/terminal-stream/client-output-queue.test.ts test/unit/server/ws-handler-backpressure.test.ts test/server/ws-terminal-stream-v2-replay.test.ts test/server/ws-edge-cases.test.ts --config vitest.server.config.ts
• FRESHELL_TEST_SUMMARY="verify server-side terminal replay batching correction" timeout 1800s npm run check