feat: Add HardSigmoid, HardSwish, and stabilized Softplus operators

[AdityaDRathore]

Port of root-project/root#20933, root-project/root#20944, and root-project/root#21092 to the standalone SOFIE repository.

HardSigmoid (ONNX Opset 6+)

• ROperator_HardSigmoid — Stateful operator with configurable alpha and beta attributes
• Formula: y = max(0, min(1, alpha * x + beta)) (defaults: α=0.2, β=0.5)
• Uses std::fmax/std::fmin with hexfloat clamp constants for AVX mapping

HardSwish (ONNX Opset 14)

• ROperator_HardSwish — Inline code generation (does not instantiate HardSigmoid)
• Formula: y = x * max(0, min(1, x/6 + 0.5))
• Split topology with intermediate variable h for debuggability
• Hexfloat constant 0x1.5555555555555p-3f for exact 1/6 representation

Softplus (Numerical Stability Fix)

• Fixes overflow vulnerability in std::log(std::exp(x) + 1) for x > 88.72
• Stabilized formula: y = (x >= 20.0) ? x : std::log1p(std::exp(x))
• Threshold defined as hexfloat 0x1.4000000000000p+4f for cross-platform exactness
• Added kSoftplus to EBasicUnaryOperator enum and GetStdLibs()

Testing

Comprehensive GTest suites exist in the ROOT PRs covering:

• Hexfloat constant verification in generated code
• Numerical correctness (linear region, clamp boundaries, overflow protection)
• Type/shape inference, error handling, StdLib dependencies

Tests can be adapted for the standalone SOFIE test infrastructure if needed.

Files changed

File	Action
src/SOFIE_core/inc/SOFIE/ROperator_HardSigmoid.hxx	New
src/SOFIE_core/inc/SOFIE/ROperator_HardSwish.hxx	New
src/SOFIE_core/inc/SOFIE/ROperator_BasicUnary.hxx	Modified — add kSoftplus
src/SOFIE_parsers/src/ParseHardSigmoid.cxx	New
src/SOFIE_parsers/src/ParseHardSwish.cxx	New
src/SOFIE_parsers/src/ParseBasicUnary.cxx	Modified — add ParseSoftplus
src/SOFIE_parsers/src/RModelParser_ONNX.cxx	Modified — register 3 operators
src/SOFIE_parsers/CMakeLists.txt	Modified — add parser sources
src/SOFIE_core/CMakeLists.txt	Modified — add operator headers