feat: add sensor observations

[han-xudong]

Summary

• add generic *_sensor station fields that resolve to rio_hw.sensors
• start configured sensor clients in Env and include their state under step.observation.sensors
• preserve sensor state through recorder / loader flattening and reconstruction
• add rio-list-sensors and station configuration documentation
• cover sensor factory, env observation, loader roundtrip, and listing behavior

Issue

Fixes #5.

Stacking

This PR is stacked on #2 (test: fix contributing test setup), which fixes #4. The branch includes #2 so the CONTRIBUTING command uv run --extra dev pytest -n 4 can be run on this feature branch. After #2 merges, this branch should be rebased onto main; the remaining diff is the sensor feature only.

Validation

• uv run --extra dev pytest -n 4 -q
• uvx ruff check docs/workflow/station_cfg.md pyproject.toml rio/_scripts/list_available.py rio/_tests/test_sensors.py rio/data/loader.py rio/envs/env.py rio/envs/factory.py rio/schema.py
• uvx ruff format --check pyproject.toml rio/_scripts/list_available.py rio/_tests/test_sensors.py rio/data/loader.py rio/envs/env.py rio/envs/factory.py rio/schema.py
• uvx typos --diff docs/workflow/station_cfg.md pyproject.toml rio/_scripts/list_available.py rio/_tests/test_sensors.py rio/data/loader.py rio/envs/env.py rio/envs/factory.py rio/schema.py

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