STARBackend: consolidate iSWAP setup state into iSWAPInformation

[BioCam]

Bundles the 7 scattered _iswap_* setup attributes into one iSWAPInformation dataclass, mirroring the existing Head96Information / MachineConfiguration pattern. No behavioral change beyond error-message wording.

Why

iSWAP setup state lived in 7 separate private attrs on STARBackend, all populated in set_up_iswap() from EEPROM, all immutable post-setup. There was no iSWAPInformation equivalent to the 96-head / machine-config records PLR already has. Costs:

• Simulation friction: chatterbox would need to set 7 attrs by hand to make iswap_request_pose work.
• Inconsistent guards: each call site had its own "is iSWAP set up?" check with a different message.
• v1b1 drift: v1b1's KX2 uses a single KX2Config record; iSWAP had no equivalent.

Changes

STAR_backend.py

• New @dataclass iSWAPInformation: fw_version, rotation_drive_x_offset, rotation_drive_y_max, link_1_length, link_2_length, rotation_drive_predefined_increments, wrist_drive_predefined_increments.
• __init__: 7 attrs → one self._iswap_information: Optional[iSWAPInformation].
• New guarded iswap_information property (raises if setup hasn't run).
• set_up_iswap(): 6 separate assignments → one iSWAPInformation(...) construction; fw_version now loaded eagerly (was lazy).
• 9 call sites migrated to self.iswap_information.*; dead lazy-fetch fallbacks removed.

STAR_chatterbox.py

• _DEFAULT_ISWAP_INFORMATION constant (Hamilton factory defaults).
• iswap_information constructor override param.
• One-line assignment in setup() when iSWAP installed.

STAR_tests.py

• TestiSWAPRequestPose._make_backend simplified (13 lines → 1).
• +5 tests: property guard (×2), chatterbox default/override/skip (×3).