Implement thermal service state-tracking (6 opcodes)

Task\n\nImplement stateful thermal service with state-tracking across all 6 opcodes:\n- get_temperature — return stored temperature per sensor index\n- set_threshold / get_threshold — persist and retrieve threshold values\n- set_variable / get_variable — persist and retrieve variable-length data with len validation\n- get_policy — return thermal policy data\n\nAdd comprehensive unit tests covering core round-trip behavior and edge cases.\n\n## Files\n- ec-service-lib/src/services/thermal.rs\n\n## PR\n- #56\n\n## Parent\nPart of #60