feat: Add centralized IEC 61131-3 type conversion layer

[marconetsf]

Summary

This PR introduces a centralized type conversion infrastructure for all plugins, addressing the issue of duplicated type conversion code across OPC-UA, Modbus, S7Comm, and future plugins (EtherCAT, etc.).

New modules added to core/src/drivers/plugins/python/shared/:

iec_type_registry.py - Centralized registry of all 22 IEC 61131-3 types:

• Complete metadata for each type (bounds, size, ctypes class, signedness, register count)
• Alias resolution (INT32 -> DINT, FLOAT -> REAL, etc.)
• Type category sets (INTEGER_TYPES, FLOAT_TYPES, TIME_TYPES, etc.)
• Size code mapping (X, B, W, D, L)

iec_type_converter.py - Unified conversion utilities:

• Value clamping with proper signed/unsigned handling
• Float <-> integer bit representation (REAL/LREAL storage)
• Modbus register combination/splitting with big/little endian support
• IEC_TIMESPEC conversions for TIME/DATE/TOD/DT types
• Default value generation per protocol
• Endianness swapping utilities

Benefits

• Eliminates ~500+ lines of duplicated code across plugins
• Ensures consistency - all plugins use the same conversion logic
• Simplifies maintenance - fix bugs in one place
• Easier to extend - adding new types requires changes in one location
• Well tested - 193 unit tests covering all edge cases

Usage example

from shared.iec_type_converter import IECTypeConverter
from shared.iec_type_registry import get_type_info

# Clamp value to type bounds
value = IECTypeConverter.clamp_to_type(1000, "SINT")  # Returns 127

# Convert float to integer representation for storage
int_repr = IECTypeConverter.float_to_int_repr(3.14, "REAL")

# Combine Modbus registers into value
value = IECTypeConverter.registers_to_value([0x1234, 0x5678], "DINT", big_endian=True)

# Get type metadata
info = get_type_info("DINT")
print(f"DINT: {info.size_bytes} bytes, range [{info.min_value}, {info.max_value}]")

Test plan

• All 193 new unit tests pass
• Pre-commit hooks pass (pylint, ruff, black, isort)
• Manual integration testing with existing plugins (follow-up PR will refactor plugins to use this)

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This is the foundation layer. A follow-up PR will refactor existing plugins (OPC-UA, Modbus) to use these shared utilities.

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