fix(session-export): wait for response pull capacity

The ZIP loop checked desiredSize only after a push and responded to an overfull queue with setTimeout(0). A timer turn does not mean the consumer drained anything, so a slow or disconnected client still allowed the producer to enqueue the complete compressed archive while later artifact and attachment reads ran eagerly.

Give the ReadableStream a 64 KiB byte queuing strategy and block the single producer on a pull-released capacity gate whenever desiredSize is non-positive. Cancellation wakes that gate through the existing producer signal; synchronous fflate output is therefore bounded to the queue high-water mark plus one input push. A regression test exhausts timer turns without consuming and proves the next media entry remains unread until response pulling begins, and the bilingual contracts now describe the real bound.

packages/host/apiproxy/README.md

@@ -28,7 +28,7 @@ Question responses are validated against their pending request before the first
 
 `session.history`'s tail page (`beforeSeq` absent) additionally carries an optional `projections` block — the watermark snapshot of every unit registered on `ctx.sessionProjections` (`@deepseek-ai/dsh-session-projection`), with `asOfSeq` = the last event seq the values reflect (`-1` on an empty log). The gateway also subscribes to the registry's change feed and mints a `session/projection` mux frame per changed unit (`{sessionId, key, value, seq}` — live push state, never logged; clients hold one generic per-session value store under higher-seq-wins). The carrier holds zero domain knowledge (each value passed its unit's own schema inside the registry; the wire schemas keep `values`/`value` wide); loadOlder pages never carry the block, and a composition without the registry serves histories without either surface.
 
-Session-log export is a host-only download surface, not an RPC: `GET /api/session.export?sessionId=…&includeDescendants=true` streams a ZIP whose files are each session's stored artifact text verbatim (the persistence backend's `readRaw` — exact durable bytes decoded from the physical encoding, never a reconstruction from parsed events), root under its original base name plus each subagent descendant under `subagents/<id>/`, and every image any included log references under `media/<attachmentId>.<ext>` (read and verified from the attachment store; a shared image appears once). Each live root or descendant crosses the authoritative `SessionStore.flush` durability barrier immediately before its raw artifact read; cold sessions have no in-memory work to flush. Compression runs on the host with fflate's streaming Zip API, so the response is chunked as it is produced and the host never holds the whole archive in one buffer, and production yields whenever the response queue fills, so a slow consumer bounds the accumulation (fflate's callback is synchronous — the drain point is the only backpressure). Request abort and response-body cancellation stop lineage and artifact work, terminate the active compressor, and propagate as cancellation rather than an HTTP 500. It requires the persistence, session-query, and attachment services: a deployment without any answers 500, a persistence backend without per-session raw artifacts answers 501, a missing root session answers 404, and a descendant without a stored artifact or a referenced image that cannot be read fails the stream (fail-loud, never silent under-export). The carrier mounts the endpoint; `ApiProxy.downloads.sessionLog` implements it.
+Session-log export is a host-only download surface, not an RPC: `GET /api/session.export?sessionId=…&includeDescendants=true` streams a ZIP whose files are each session's stored artifact text verbatim (the persistence backend's `readRaw` — exact durable bytes decoded from the physical encoding, never a reconstruction from parsed events), root under its original base name plus each subagent descendant under `subagents/<id>/`, and every image any included log references under `media/<attachmentId>.<ext>` (read and verified from the attachment store; a shared image appears once). Each live root or descendant crosses the authoritative `SessionStore.flush` durability barrier immediately before its raw artifact read; cold sessions have no in-memory work to flush. Compression runs on the host with fflate's streaming Zip API, so the response is chunked as it is produced and the host never holds the whole archive in one buffer. Once the response queue reaches its 64 KiB byte high-water mark, production waits until consumer pull restores positive capacity; fflate's synchronous callback can overshoot that bound only by the output of one bounded input push. Request abort and response-body cancellation stop lineage and artifact work, terminate the active compressor, and propagate as cancellation rather than an HTTP 500. It requires the persistence, session-query, and attachment services: a deployment without any answers 500, a persistence backend without per-session raw artifacts answers 501, a missing root session answers 404, and a descendant without a stored artifact or a referenced image that cannot be read fails the stream (fail-loud, never silent under-export). The carrier mounts the endpoint; `ApiProxy.downloads.sessionLog` implements it.
 
 Session titles ride the generic projection pair like every other domain — the history-tail `projections` block plus `session/projection` frames under the `title` key. Titles do not join `session.list`; cold sessions remain metadata-only there until opening or resuming attaches their logs. `session.rename` accepts an explicit user title (resuming a cold session first), delegating to `ctx.sessionTitle.rename` — the accepted `session/title` event pins the title against automatic regeneration — and returns the normalized title plus its event seq so a client settles its `title` projection cell ahead of the push frame; a title that normalizes to empty returns `title-invalid`.