fix: implement sign-magnitude encoding for signed registers

[jimsynz]

Summary

• Implements sign-magnitude encoding for Feetech servo signed registers (previously using two's complement which was incorrect)
• Adds comprehensive test coverage for encoding/decoding signed values

Details

Feetech servos use sign-magnitude encoding (not two's complement) for signed values, with different sign bit positions:

Register Type	Sign Bit	Valid Range
position_offset	Bit 11	-2047 to +2047
goal_position	Bit 15	-32767 to +32767
present_position	Bit 15	-32767 to +32767
present_speed	Bit 15	-32767 to +32767
present_load	Bit 10	-1023 to +1023

Changes

Protocol module:

• Added encode_sign_magnitude/3 and decode_sign_magnitude/2 functions

ControlTable module:

• Added :position_signed conversion type
• Fixed :speed_signed and :load_signed to use sign-magnitude
• Added {:sign_magnitude, bit} conversion for raw signed values

STS3215 control table:

• Updated goal_position and present_position to use :position_signed
• Updated position_offset to use {:sign_magnitude, 11}

Tests added

• 12 new tests for encode_sign_magnitude/3 and decode_sign_magnitude/2 in protocol_test.exs
• 12 new tests for signed value conversions in sts3215_test.exs (negative positions, speeds, loads, position offsets)

Test plan

• All 90 tests pass
• Formatter check passes
• Credo check passes
• Dialyzer check passes