[moetemp]

What changed?

This PR adds Producer scopes. A scope is the Producer of one Activity attempt or one client, with one writer for each Stream, so lang keeps only a scope id.

• OpenActivityScope derives the Producer from the attempt's ActivityContext. From attempt 2, it opens a writer on each Stream in the attempt's arguments, so the attempt withdraws the earlier attempts' Records even if it writes nothing.
• OpenClientScope is client:<identity>:<key> at epoch 1, with a key from the runtime's random key and a counter. With a name it is client:<name> at the given epoch. A forked Python child builds a new runtime, so it gets a new key with no fork hook.
• Publish opens the scope's writer on first use and writes. CloseStreamAs closes that writer and then the Stream, as the scope's Producer. An Activity scope refuses it with NOT_OWNER and calls no Store.
• CloseScope closes the writers and reports a failed one in its answer. FreeScope drops a scope with no wait, for a finalizer.
• A dropped OpenActivityScope or CloseScope releases the writers that the scope holds.
• The writer table counts lang's handles and the scopes' handles apart. CloseWriters and Shutdown release lang's only, since another Worker of the client may still use a scope's writer, and a scope releases its own when it ends. A late FreeWriter from lang takes no scope handle.
• Shutdown closes one scope at a time. A dropped one puts back the scope that it was closing, and the next Shutdown ends it. While it runs, the Free methods still work, so lang can end a buffer whose pump is gone. After it, only Shutdown, the static methods and the Free methods run, and the Free methods do nothing. A scope or writer that opens during it is freed and refused.
• A writer's retry duration counts as at most a day, so the retry math can't overflow.
• CloseStreamAs holds the Stream's open lock while it ends the writer, so a concurrent first publish can't share that writer.
• Flaky, the fault-injecting memory Store of the writer suite, moves to the suites' support module for the scope tests.
• The scope suite runs through the byte call, as lang sends it.

Depends on #221 (K11). Review lane is the Rust reviewers.
Part of the Brook review series.

Why?

The Activity interceptors of sdk-python, and later of TypeScript and .NET, then fill one ActivityContext and keep one id. The Producer rule, the attempt-start withdrawal and the writer table live here once.

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 scope 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.