[moetemp]

What changed?

This PR adds the Stream service WIT, the input of each SDK's nexgen client.

• stream-service.wit is a System Nexus service. There's no writer WIT. SDKs call the writer methods with runtime.proto messages, so no SDK generates writer bindings with nexgen.
• Each type binds to its split A1 proto. stream-ref.owner is an execution record.
• The read item stream-item is a variant bound to A1's StreamRecord.item oneof, with the cases data, superseded and close. stream-error-reason binds A1's error enum.
• A read record's item is optional, so an item kind that only a newer server knows decodes as no item.
• Each request's %stream is required, and so are the lists. Each record lists its required fields first, so the generated Python models stay positional, with no kw_only.
• The resource methods are publish, read, close and describe, the names users call. @nexus.operation keeps the wire names.
• close-with-run, and the close's failed-tasks and previous-completed-task, are @nexus.omit, like the other native-only fields.
• Unlike moetemp/sdk-streams#52, it follows the reworked nexgen. ReadRecords uses @nexus.poll, and create-stream returns the stream resource with created on it.
• A create of a closed, expired or other-owned Stream fails, so the resource takes no create status. read takes max-bytes, which the request has. Only the client's read-records takes last, since Workflow replay needs a fixed range.
• The WIT takes the nexgen directives of moetemp/nexgen#78 to #97, so the generated Stream API is the user API. Stream is generic in its value type body-t, and a create takes a StreamDefinition. The fields that the runtime sets have a @nexus.source per target, or @nexus.api-omit. The create and the handle factory take id and backend, not the ref.
• publish(*values) returns the last offset. The read poll yields each Record and resumes after the last one, async with closes the Stream, and each error reason with a class raises that class.
• The close has no status in the API. K6 gives CANCELED for a failure with CanceledFailureInfo.

Depends on #219 (K9), but needs only #212 (K1). Review lane is the proto and WIT reviewers.
Part of the Brook review series.

Why?

The WIT is only a nexgen input, so it's last and alone. After review the crate ships with the sdk-rust crates. sdk-python moves its pin to this PR's head in its coming repin.

How did you test it?

cargo fmt, cargo clippy -- -D warnings (with and without features), cargo test -p temporalio-streams --all-features and cargo doc pass at this commit.
nexgen at moetemp/nexgen#97 (c88f90ef) generates the WIT for Python, TypeScript and .NET.

[moetemp]

Closing. The Stream service WIT moved to temporalio/api#895.