STAR save_configuration: firmware_variable values written beside the fields

[BioCam]

Since #1458 an X-arm's current_limit_range and current_limit_digits are properties read off its firmware_version, so save_configuration no longer writes them: dataclasses.asdict takes fields only. A saved configuration then shows firmware_version but not the limiter range the driver will enforce for it, and the recordings could not keep the range either - test_every_key_is_a_field refuses a key nothing reads back.

• a configuration lists the properties its firmware version decides in firmware_variable, a ClassVar: never a field, fine on a frozen dataclass
• save_configuration writes them in a firmware_variable block beside the fields, nested configurations included; reading back ignores the block and the values come from the code
• XArmConfiguration.firmware_variable = ("current_limit_range", "current_limit_digits")
• the five recordings carry the block under device.left_arm; design.md P13 says what it is

"left_arm": {
  "firmware_version": "5.0S a 2012-04-03 (X0 XE167)",
  "current_limit_default": 7,
  "firmware_variable": {"current_limit_range": [0, 15], "current_limit_digits": 2}
}

Tests: master_tests - every firmware_variable block in a recording holds exactly what the code gives, nothing missing or extra; keys_no_field_reads passes the block and nothing else; a saved arm carries its block.

🤖 Generated with Claude Code