ci: Remove generated conformance bottlenecks

[zharinov]

The Rust-only conformance harness mixed VM oracle generation, Rust emission, golden compile checks, and execution by compiling a 164k-line synthetic program inside the normal test and coverage runs. This removes that harness instead of carrying its shape into future cross-platform conformance work.

• Remove the generated matcher conformance test, its shared fixture support, and the combined Rust-section compile harness.
• Rename macro_ui to macro_diagnostics; stable CI still runs the full compile-pass/fail contract, while coverage skips its nested compiler invocation.
• Keep nightly coverage and the three #[coverage(off)] exclusions. Remove only invalid nested feature gates; the crate-root gate remains.
• Drop the redundant standalone plotnik-lib WASM check and the LLVM install that the runner reports as already satisfied.
• Stop Codecov from searching for unrelated coverage formats when lcov.info is supplied explicitly.

This intentionally removes exhaustive generated-Rust/VM conformance until a fixture-output-based, target-neutral conformance pack replaces it. The VM fixture corpus, macro integration tests, and macro diagnostics remain.

On the same checkout, a warmed full test run fell from 53.85s to 34.95s. With compiler-based diagnostics excluded from instrumentation, a subsequent nightly coverage run produced lcov.info without Rust warnings in about 66s.