Add ValueInt.from_bits() for signed/unsigned bit pattern conversion

[Copilot]

Hardware interface values stored in ValueInt are misinterpreted in Python—bit patterns from DUT registers are always treated as unsigned, breaking signed value semantics.

Changes

API Addition (src/vsc/model/value_scalar.py)

• Added ValueInt.from_bits(value, width, signed=False) class method
• Masks input to specified bit width
• Applies two's complement conversion when signed=True and MSB is set

Tests (ve/unit/test_value_int.py)

• Covers 1/4/8/16/32-bit widths, masking, sign detection, edge cases

Documentation (doc/source/data_types.rst)

• New section: "Converting Bit Patterns to Values"

Usage

import vsc

# DUT returns 0xFFFFFFF0 from 32-bit signed register
dut_val = read_register()
python_val = vsc.ValueInt.from_bits(dut_val, width=32, signed=True)
# Result: -16 (not 4294967280)

# Unsigned interpretation
vsc.ValueInt.from_bits(0xFF, width=8, signed=False)  # 255

# Signed interpretation
vsc.ValueInt.from_bits(0xFF, width=8, signed=True)   # -1

Addresses #248

Original prompt

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This section details on the original issue you should resolve

<issue_title>ValueInt class needs something like $signed method in SV</issue_title>
<issue_description>So im using the value stored in an ValueInt object in python and the stored binary value which works correctly with dut interface always gets misinterpreted in Python world because python infers the given numbers as unsigend. I think having a $signed helper method in ValueInt class can be helpful to prevent such problems. I have currently implemented a $signed function in this way:

def signed(numb, width):
if (numb >> (width-1)):
return(-((2 ** width) - numb))
else:
return numb

</issue_description>

<agent_instructions>Implement 'from_bits' as described in #248. Use the algorithm from the initial comment. Write tests to ensure value conversion (specifically, value masking, sign detection, and conversion) are implemented properly.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@mballance Useful idea, @Amirk97 . I'm wondering whether it would make sense to have this be a way to construct a ValueInt. For example:

dut_val = <path>
vsc_val = vsc.ValueInt.from_bits(dut_val, width=32, signed=True)

Thoughts?</comment_new>

• Fixes ValueInt class needs something like $signed method in SV #248

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