feat(transport): keep large messages intact across relay connections (#8302)

Negotiate shared binary chunk framing below application messages and routing, with bounded logical assembly and paced writes that respect encrypted relay limits. Preserve legacy admission, reconnect state, sibling route isolation, and native single-frame upload behavior.

docs/protocol-compatibility.md

@@ -52,6 +52,33 @@ On capable daemons, connecting creates no timeline or event demand. Client subsc
 release it on unsubscribe, and restore it after reconnect. Raw message observers inspect traffic
 without requesting streams. Application caches and which agents are visible remain caller-owned.
 
+## Binary chunking
+
+`binary_chunking_v1` in hello and optional `server_info.features.binaryChunkingV1`
+negotiate connection-scoped framing. Application serializers and routed envelopes use
+the same boundary. Small messages and handshakes keep their existing representation.
+Older peers retain existing frame limits.
+
+The reserved binary opcode `0x31` is followed by version `1`, original kind
+(`0` text, `1` binary), and START/END flag bits `0`/`1`. Remaining bytes are an
+unchanged slice. There is no total length or chunk count; a future receiver can
+consume slices incrementally. Today the receiver buffers up to the existing
+100 MiB logical limit and decodes UTF-8 only after END. Transfers cannot interleave.
+Disconnect clears partial state and negotiation.
+
+The lower transport supplies its plaintext frame budget, including encryption
+overhead. The sender waits for drain between chunks while ordinary messages keep
+the existing delivery path. Slow-client protections count messages queued behind
+an active transfer as well as bytes already in the socket.
+The framing module never selects an encryption construction.
+
+Native uploads retain the platform's existing queue limits. React Native's
+WebSocket does not report its queued bytes, so the client cannot wait for native
+drain. Android's installed OkHttp transport closes when a send would put more
+than 16 MiB in its queue. The native transport disables outbound chunking, including
+on routed channels, and retains its existing single-frame admission errors.
+Inbound chunking remains enabled; daemon-to-native responses use the daemon's paced writes.
+
 ## Owned observations
 
 `owned_subscriptions` and `server_info.features.ownedSubscriptions` negotiate the source-owned