Fix: Allow writing -1 to register 2704 (MaxDischargePower) #1580

[drurew]

Description

This PR addresses Issue #1580 where writing -1 to register 2704 was treated as a scaled value (resulting in -0.1 or invalid values) instead of the special 'unlimited' flag.

Changes

• Introduced a new Modbus type int16_ul (int16 unlimited) in mappings.h and mappings.cpp.
• int16_ul behaves exactly like int16 (with scaling), EXCEPT:
  • Read: If the DBus value is -1.0, it reports as 0xFFFF (-1).
  • Write: If the Modbus value is 0xFFFF (-1), it writes -1.0 to DBus (bypassing scaling).
• Updated attributes.csv to use int16_ul for register 2704 (MaxDischargePower).

Verification

• Verified logic via standalone script simulating the getValue and setValues transformations.