CI: Add GitHub Actions workflow for conformance tests and performance benchmarking

[simonCatBot]

Summary

Replaces legacy Travis CI and GitLab CI with a GitHub Actions workflow modeled after kiritigowda/rustVX/.github/workflows/conformance.yml.

What this PR does

• Deletes .travis.yml and .gitlab-ci.yml
• Adds .github/workflows/ci.yml with three phases:

Phase 1 — Build (parallel)

• build-debug: Debug build with all extensions + coverage flags (-fprofile-arcs -ftest-coverage)
• build-release: Release build with all extensions (for benchmarking)
• build-cts: Build Khronos CTS against the debug install

Phase 2 — Test (parallel, needs build-cts)

• cts-baseline: Smoke tests + code coverage collection
• cts-enhanced-vision, cts-neural-networks, cts-ix, cts-graph-features, cts-data-objects, cts-user-kernels: Stage-based CTS runs with --filter, matching the rustVX conformance pattern

Phase 3 — Performance gate (PR only, needs build-release + build-main)

• build-main: Build the merge target (main / openvx_1.3) release for comparison
• perf-gate\: Clone openvx-mark`, build against PR and main releases, run benchmarks, compare results

Design decisions

• Separate debug/release builds: Debug is instrumented for coverage and CTS; Release is optimized for openvx-mark benchmarking
• Artifact reuse: Build once, test/benchmark many times — avoids rebuilding the library N times
• chmod +x after artifact download: actions/download-artifact@v4 strips executable permissions
• continue-on-error: true for known pre-existing failures (not regressions):
  • cts-neural-networks: AlexNetTestNetwork fails to load weights (missing test_data file)
  • cts-graph-features: GraphPipeline.* fails (pipelining incomplete in C model target)
• Excluded: --conf_nnef due to NNEF-Tools C++ parser GCC 11 incompatibility

Notes

This PR is intentionally separate from #60 (bug-fix PR) so CI infrastructure can be reviewed and merged independently.

cc: @kiritigowda — modeled after rustVX conformance workflow, thanks for the reference!

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