[moetemp]

What changed?

This PR adds the stream handles an Activity and a client use to read and write a Workflow's stream through Core.

• get_stream_handle() and activity_handle() return a StreamHandle with read, latest and producer.
• A producer hashes before the codec, encodes the bodies and calls the generated append. It moves its sequence only when an append succeeds.
• An Activity sends a typed ActivityProducer and Core derives <activity id>@<scheduling run id>, so every SDK writes the same producer id (DD-44).
• Every read follows the run chain across Continue-as-New and ends only with the chain. chain_run_id= opens an older chain of a reused Workflow id. An Activity's handle names its scheduling run's chain, so its appends land there.
• Reads are long polls that hand back Core's state. Both sides use the Workflow's WorkflowSerializationContext, so a context-aware codec decodes the records.
• A Local Activity gets StreamUnsupportedError. Its attempt isn't durable, so its first append after a crash would be refused as stale.
• The changelog fragment says what Brook ships. Activities and clients append, and outside readers follow from a cursor.

Why?

Core owns owner checks, cursors, dedupe and supersession. So a handle is a thin loop over the stub with the codec on both sides (DD-88).

How did you test it?

Link to a test plan if any -

• Unit Tests
• Staging
• End to End Tests

tests/contrib/streams/test_outside_handles.py runs on the dev server.

[moetemp]

Combined into #241.