Add rounding direction setting

[Patrick1610]

Summary

• Add a rounding_direction option to initial setup and Options Flow.
• Preserve existing behaviour with nearest as the default and fallback for old configs.
• Apply floor/nearest/ceiling rounding to the final raw setpoint, with epsilon handling for exact boundaries.
• Expose rounding diagnostics and update README examples for Plugwise Emma.
• Extend coordinator and sensor tests for rounding directions and diagnostics.

Validation

• python3 -m py_compile for integration and tests passed with PYTHONPYCACHEPREFIX=/private/tmp/climatesync-pycache.
• python3 -m json.tool passed for both translation files.
• git diff --check passed.
• Self-contained coordinator sanity check passed for 19.1 + 0.1 -> raw 19.2 -> 0.5 ceiling -> 19.5.

Note: local environment does not have pytest, homeassistant, or voluptuous, so the full pytest and HA import runs could not be executed locally.