ci: add full OpenVX conformance workflow and conformance coverage doc

Adds .github/workflows/ci.yml that builds rustVX and the Khronos OpenVX
CTS with every supported feature flag enabled (Vision, Enhanced Vision,
Neural Networks, NN/16, NNEF Import, Import/Export IX, Pipelining,
Streaming, User Data Object, U1). The workflow splits the resulting
CTS suite into the same broad bands used by KhronosGroup/OpenVX-sample-impl:

- baseline / smoke / coverage
- vision + vision kernels + enhanced vision
- neural networks
- NNEF import
- import/export (IX)
- graph features (delay, ROI, callbacks, pipeline, streaming)

New conformance areas that exercise known gaps in rustVX are marked
continue-on-error until the implementation is complete.

Adds CONFORMANCE.md documenting the matrix of what is covered, what gaps
remain, and how to run the full suite locally.

This is complementary to the existing conformance.yml workflow; it does not
remove or change any existing job.

Author: Simon