.github/workflows/standards-build.yml

@@ -197,3 +197,60 @@ jobs:
         env:
           RUSTFLAGS: ${{ matrix.rustflags }}
         run: cargo run -q -p ${{ matrix.conformance }} --bin ${{ matrix.conformance }}
+
+  # ── The macOS energy backend is exercised NOWHERE otherwise ───────────────
+  #
+  # `jcp-energy`'s IOReport meter — the Apple Silicon rung of the energy
+  # ladder — is behind `#[cfg(target_os = "macos")]`, and `jcp-rs` is pinned to
+  # the linux runner above. A platform-gated test plus a single-platform runner
+  # assignment is zero coverage, and it is indistinguishable from passing: the
+  # code compiles out, the suite reports green, and nothing has been measured.
+  # Same family as a feature-gated test with no `--features`, with `target_os`
+  # as the gate.
+  #
+  # This is not hypothetical. On 2026-09-19 the counter read 0 µJ across a full
+  # run on the dev Mac — `IoReport::open()` succeeding while `read_uj()`
+  # returned nothing — and by 2026-09-21 it had healed with no code change in
+  # between. Both observations came from a human running the suite by hand;
+  # CI saw neither. A meter that silently reads zero and later recovers is
+  # exactly the failure a human spot-check is worst at catching.
+  #
+  # Deliberately narrow: `-p jcp-energy` only. `jcp-rs` stays on linux, so this
+  # ADDS the platform that was missing rather than trading one for the other,
+  # and it costs one small crate rather than a second full workspace run.
+  #
+  # Verified to catch the real symptom: forcing `read_uj()` to return `Some(0)`
+  # makes `cumulative_advances_after_cpu_burn` fail with
+  # `energy must advance: before=0 after=0` — byte-identical to what was
+  # observed on 2026-09-19.
+  macos-energy-backend:
+    name: macOS energy backend (jcp-energy)
+    runs-on: macos
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Rust (workspace pin)
+        shell: bash
+        run: |
+          export PATH="$HOME/.cargo/bin:$PATH"
+          command -v rustup >/dev/null 2>&1 || \
+            curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --profile minimal
+          echo "$HOME/.cargo/bin" >> "$GITHUB_PATH"
+
+      - name: Cache cargo registry + target
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            jcp/jcp-rs/target
+          key: standards-jcp-energy-macos-${{ hashFiles('jcp/jcp-rs/Cargo.lock', 'rust-toolchain.toml') }}
+          restore-keys: |
+            standards-jcp-energy-macos-
+
+      # `--lib` keeps this to the crate's own tests, which is where every
+      # `cfg(target_os = "macos")` block in this crate lives.
+      - name: Energy backend tests (the cfg(macos) blocks)
+        working-directory: jcp/jcp-rs
+        run: cargo test --lib -p jcp-energy
+