CI: rendering / test-support integration tests never run in CI

[yfedoseev]

Summary

The entire feature = "rendering" integration-test tier is never executed in CI. This is pre-existing (independent of any single PR) and means rendering regressions can land on main undetected.

Evidence

• Cargo.toml:314 — default = ["icc", "legacy-crypto"]. rendering and test-support are not default features.
• The only full test step is .github/workflows/ci.yml:162 — cargo test --verbose — runs under default features only, so every #![cfg(all(feature = "rendering", ...))] test file compiles to 0 tests.
• main already has 10+ rendering-gated test files silently skipped: test_render_output_intent, test_jbig2_rendering, test_pdfa_roundtrip, test_render_ocg_filtering, test_render_real_world_fixture, test_render_resolution_pipeline_qa_wave1/2, test_concurrent_document_reads, test_debug_visualization, test_render_region_and_fit.
• grep 'cargo test.*rendering' .github/workflows/ci.yml → no matches. test-support appears nowhere under .github/.
• The "extended" build job that enables rendering only runs cargo build, never cargo test.

Impact

A green CI does not prove the rendering test suite passes. Large new rendering test suites (e.g. the §11 transparency work in #674, ~19k lines / 260 tests) fall straight into this dead zone.

Suggested fix

- run: cargo test --features rendering,test-support
- run: cargo test --features rendering,test-support,icc-lcms2   # separate job; installs Little CMS 2