[moetemp]

What changed?

This PR adds buffered publishers. The runtime keeps the batching rules, and lang keeps only its codec, which runs once per batch.

• A batch is due max_delay after its first value, at once when it has max_batch values, or at once for a flush. One batch is in flight at a time. After a failed batch, the buffer writes nothing more, so a reader sees no gap.
• With no codec, lang opens the buffer with encoded, and the runtime writes each batch itself on a Tokio task.
• With a codec, lang runs a small pump. It takes each batch with BufferNextBatch, runs the codec, and gives the batch back with BufferWrite, or the codec's error.
• BufferPush returns at once, and fails with the buffer's error after a failed batch. BufferFlush waits for every value pushed before it, and answers with the last offset, the unwritten count and the failure.
• CloseScope and Shutdown flush and free the scope's buffers first. They report a buffer failure that lang didn't get yet. During Shutdown, lang's pump calls still run. A push during the scope's close fails, so the close drops no value.
• A BufferWrite writes on a task of its own, so a dropped call doesn't leave its batch in flight. A batch that a dropped pump took but never wrote comes again on the next BufferNextBatch. Records of another count than the batch fail the call and the buffer. A freed buffer gives its pump done, and counts a taken batch as unwritten.
• A batch that the writer splits and whose later part fails counts only that part as unwritten, and keeps the first part's offset.
• A write task that ends early, such as by a panic, fails its batch, so a flush still ends.
• The crate's tokio dependency takes the macros feature, for select!.

Depends on #224 (K14). Review lane is the Rust reviewers.
Part of the Brook review series.

Why?

BufferedPublisher has subtle timer, batch and failure rules, and every SDK needs the same ones. A remote codec costs a round trip per call, so the codec stays in lang and runs once per batch.

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. The buffer suite runs on the memory Store here, and on Redis in R4 (temporal-community/sdk-streams#4).

[moetemp]

Parked. Nothing calls these runtime methods yet, since sdk-python does this work in Python. The branch stays, and this layer comes back with its first caller, for example a TypeScript or .NET port or Rust read handles.