Add Tether Improve worker dry-run entrypoint

[rylinjames]

Summary

• add stdlib-only Improve worker entrypoint that consumes fastcrest.improve.worker_input.v1
• materialize only selected bounded evidence and emit deterministic worker_result.v1 envelopes
• add typed failure envelopes for malformed, insufficient, unselected, expired, deleted, or do_not_train evidence
• export run_improve_worker for local/Cloud adapter integration

Validation

• PYTHONPATH=src pytest tests/test_improve_worker.py tests/test_finetune.py tests/test_snapflow_backend.py -q -> 59 passed, 4 skipped
• PYTHONPATH=src ruff check src/tether/finetune/improve_worker.py src/tether/finetune/init.py tests/test_improve_worker.py -> passed
• PYTHONPATH=src pytest tests/test_agent_client.py tests/test_agent_daemon.py tests/test_agent_commands.py tests/test_agent_config.py tests/test_agent_hardware.py tests/test_improve_worker.py -q -> 41 passed
• git diff --check -> passed

No credentialed GPU/Modal smoke was run; this is the Tether dry-run worker and contract-compatible envelope path.