[moetemp]

What changed?

This PR adds StreamRecord and the hashes lang takes before Core stores the records.

• StreamRecord[T], RecordKind, Supersession and Cursor go in temporalio.contrib.streams.
• _body.py takes the plaintext content_hash and the batch digest, both before the codec. Core stores them unchanged.
• Only the writer that made the digest compares it. Deterministic protobuf bytes differ across implementations.
• _wire.py turns a record of Core's read answer into a StreamRecord. A supersession that Core synthesized becomes a SUPERSEDED record.
• The envelope proto is Core's, so this PR has no proto of its own.

Why?

Only lang has the plaintext. With both hashes taken before the codec, a retry through a codec with a fresh nonce still dedupes (DD-11).

How did you test it?

Link to a test plan if any -

• Unit Tests
• Staging
• End to End Tests

tests/contrib/streams/test_records_wire.py also checks the digest against the vector Core's tests pin.

[moetemp]

Combined into #239.