Attaching to a large session can stall for ~100 s on the last replay chunk #458

What happens

Attaching to a large session sometimes takes about 100 seconds. The history arrives in chunks: the first chunks land in tens of milliseconds, then the client waits about 100 s for the last one.

Why it matters

The session looks empty or frozen to the person attaching (web UI, TUI) for over a minute and a half. It hits large sessions, which are the ones people come back to.

What was seen

During the audits for #457 (lazy session loading), the measurement auditor saw it on main (aed1afb) as well as on the branch, so the branch didn't cause it:

• An attach to a 5.7 MB / 23 MB session took about 101 s in 2 of 7 benchmark runs.
• On main: chunks f0 to f4 arrived within 80 ms, and the final 218 KB chunk arrived at 101,929 ms.
• The final chunk waited on the socket's drain callback, which didn't fire. Something else released it about 100 s later.

What we want

An attach replay finishes as soon as the client can take the data, and never waits on a drain that doesn't come. If the socket is genuinely stuck, the replay should fail visibly rather than stall.

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🤖 Implementation context (for agents / maintainers)

• Where: src/daemon/server.ts around the chunked replay flush (≈ lines 987-997 on aed1afb), which awaits the socket's drain before sending the next chunk.
• Likely cause: Bun's ServerWebSocket.send backpressure status vs. when drain fires. If the last send returned a backpressure status but the buffer emptied without a drain event (or the wait was armed after it fired), the await only ends when something else (a timeout, the next frame) unblocks it. The ~100 s may match an idle or ping timeout.
• Reproduction: the auditor's stall.ts (per-frame timing of an attach) against a daemon holding sessions with multi-MB transcripts; repeat attaches until one stalls.
• Fix direction: re-check getBufferedAmount() on a short timer instead of relying only on drain, or arm the wait before checking the send status; bound the wait and close the socket with a clear code if it never drains.