CI: feature matrix, run clippy, deny rustdoc warnings

[shinaoka]

Part of #199 (Phase 3, CI). All file:line references are pinned to origin/main at 9da9b9f.

Today .github/workflows/ci.yml runs cargo fmt --check, cargo test (default features only), facade README doctests, coverage, and cargo doc. Consequences measured in the audit:

• The clippy component is installed (ci.yml:32) and never run.

• No feature matrix: parallel is never compiled in CI, so every parallel branch (including the whole parallel uninit family) is untested; a no-default-features lane is also missing. (The blas lanes concern leaves with the einsum2 retirement; until Phase 2 lands, note that the Add safe uninitialized GEMM overwrite paths #196/Fix uninitialized GEMM parallel bounds #197 uninit GEMM code is never compiled in CI because the default backend is faer.)

• The doc job passes with broken intra-doc links (no -D warnings).

• Add matrix lanes: default, parallel, no-default-features (and parallel together with simd off).

• Run cargo clippy --workspace with a deny policy consistent with the repository rules.

• Set RUSTDOCFLAGS: -D warnings on the doc job.

• Keep the per-file coverage gate as is.