refactor: rename working memory internals and bump version

[rlippmann]

What changed

• Rename the private Python _State implementation type to _WorkingMemory.
• Rename the private Engine._state attribute to Engine._working_memory, including its Python-specific defensive-path test.
• Update the root README to introduce working memory as the host-side premise and policy concept.
• Bump the package version from 0.9.1 to 0.9.2 in pyproject.toml and uv.lock.

Why

The internal names now match the architecture terminology without changing public APIs or runtime behavior. The patch version bump prepares the next release.

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)
• git diff --check passes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (6719539) to head (9bb2ac5).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #281   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          773       773           
=========================================
  Hits           773       773           

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