Fix GPU shader correctness and warnings surfaced by DXC warning flags

[num3ric]

• GpuShaderUtils: add intCast() helper that emits a C-style cast on HLSL (silences -Wfloat-conversion) and a functional cast on GLSL/MSL/OSL.
• FixedFunctionOpGPU: use intCast() for the three implicit float->int conversions in the ACES2 hue-weight setup and cusp-table search.
• FixedFunctionOpGPU: drop the unused _Add_SinCos_Shader calls in Gamut_Compress_Fwd/Inv_Shader (gamut-compress stays in JMh space, so cos_hr/sin_hr were dead).
• GradingBSplineCurve: in the HUE-FX inverse path (curveIdx == 7), offset knStartY by knStart to match the CPU evalCurveRevHue.
• GradingRGBCurveOpGPU: inverse eval no longer declares identity_x (it's never used in the inverse), and the in/out qualifier branch is folded into a single emitter — matches the CPU evalCurveRev signature.
• DxApp: when verbose (test_gpu_exec --dx -v), pass -Wall -Wextra -Wconversion to DXC and print DXC_OUT_ERRORS even on success, so generated-shader warnings are visible during debugging.

[num3ric]

See attached file for warning logs (without fixes included here): dxc_warnings_full.log