feat: stream composition API — select, schema, relay filtering
1. Receiver-side `select` clause:
- `stream from "url" { select: field1, field2 }`
- Parser, AST, codegen all updated
- Emits: `_connectStream(url, ["field1","field2"])`
- Client-side _csFilter strips unwanted fields
2. Schema announcement (0x32):
- Sources send output schema on connect
- Lists registered signal names and mode
3. Relay schema cache:
- ChannelState stores schema from 0x32
- Forwarded to late-joining receivers
4. Relay-side subscribe filter (0x33):
- Receivers send wanted fields after connecting
- Relay strips unwanted JSON keys from 0x30/0x31
frames before forwarding — saves bandwidth
Protocol: SchemaAnnounce=0x32, SubscribeFilter=0x33
54 tests pass, all crates build clean.
a8235c48b3