CI: add cargo clippy lint job to build.yml

[dobby-coder]

Problem

.github/workflows/build.yml runs cargo test and cargo fmt --check per-workspace, but there is no cargo clippy job. Clippy catches whole classes of bugs (correctness, perf, suspicious patterns) that rustc and rustfmt miss, and is the standard third leg of a Rust CI matrix.

Adding it now should be cheap; running it for the first time will surface a backlog of warnings to triage.

Suggested job (place alongside format: in build.yml)

  clippy:
    name: Clippy workspace
    strategy:
      matrix:
        workspace: [core, pkg, cli, ffi]
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: dtolnay/rust-toolchain@stable
        with:
          components: clippy
      - run: cargo clippy --manifest-path pg-${{ matrix.workspace }}/Cargo.toml --all-targets --all-features -- -D warnings

pg-core features differ — if --all-features is too aggressive there (e.g. enables test + rust + stream simultaneously), drop to the feature set the test: job uses for core (test,rust,stream).

Rollout

1. Land the workflow first with -- -W warnings (warn-only) so it doesn't block PRs while the backlog is triaged.
2. Fix or #[allow(...)] the warnings (separate PR).
3. Flip to -D warnings once green.

Verification

• The job appears in the next PR's checks list.
• Clippy completes for all four manifests on ubuntu-latest.

---

Filed by the audit routine. Comment to triage.