fix(fn-dsa): shake256x4 could not be built per-crate — missing feature forwarding

.github/actions/test-fn-dsa/action.yml

@@ -109,6 +109,38 @@ runs:
         cargo check --no-default-features --features "alloc,rand" --target thumbv7em-none-eabi
         echo "✅ no_std check completed"
 
+    # Per-crate feature selection. Every other feature gate in CI is workspace-wide
+    # (`--all-features`), where the `lib-q-fn-dsa` facade enables `shake256x4` on `-comm` and
+    # feature unification hands that same `-comm` build to every leaf — so a leaf whose own
+    # `shake256x4` forwards nothing still compiles and the missing forward is invisible.
+    # `-kgen`/`-sign` shipped exactly that (`shake256x4 = []`, as upstream fn-dsa 0.3.0 does):
+    # the feature flipped their call sites onto `fn_dsa_comm::shake::SHAKE256x4`, which `-comm`
+    # only compiles under the same feature, so `cargo check -p lib-q-fn-dsa-kgen --features
+    # shake256x4` failed with E0433 for every consumer that selected the crate directly.
+    # Keep one `-p` per invocation: merging them re-unifies the features and tests nothing.
+    - name: Per-crate feature wiring (shake256x4)
+      shell: bash
+      run: |
+        echo "🔗 Checking per-crate feature wiring (no cross-crate feature unification)..."
+        cd "${{ github.workspace }}"
+        for p in lib-q-fn-dsa-comm lib-q-fn-dsa-kgen lib-q-fn-dsa-sign; do
+          cargo check -p "$p" --features shake256x4
+        done
+        for p in lib-q-fn-dsa-comm lib-q-fn-dsa-kgen lib-q-fn-dsa-sign lib-q-fn-dsa-vrfy; do
+          cargo check -p "$p" --all-features
+        done
+        # Manifest-level guard for the same invariant (fails without building the broken config).
+        cargo test -p lib-q-fn-dsa-kgen --test feature_forwarding
+        # SHAKE256x4 must equal four 64-bit-interleaved SHAKE256 streams on BOTH dispatch
+        # paths; x86_64 takes the AVX2 branch unless no_avx2 forces the portable one.
+        cargo test -p lib-q-fn-dsa-comm --features shake256x4 shake256x4
+        cargo test -p lib-q-fn-dsa-comm --features shake256x4,no_avx2 shake256x4
+        # Exercises the feature through keygen/signing (gauss::sample_tv carries its own
+        # shake256x4 test-vector set).
+        cargo test -p lib-q-fn-dsa-kgen --features shake256x4
+        cargo test -p lib-q-fn-dsa-sign --features shake256x4
+        echo "✅ Per-crate feature wiring verified"
+
     - name: Run quick FN-DSA smoke tests
       shell: bash
       run: |

lib-q-fn-dsa/fn-dsa-kgen/Cargo.toml

@@ -27,4 +27,9 @@ num-traits = { workspace = true }
 [features]
 default = []
 no_avx2 = []
-shake256x4 = []
+# MUST forward to `-comm`: the type this crate switches to under the feature
+# (`fn_dsa_comm::shake::SHAKE256x4`) is itself `#[cfg(feature = "shake256x4")]` inside
+# `-comm`. Declared empty (upstream fn-dsa 0.3.0 does the same), `cargo check -p
+# lib-q-fn-dsa-kgen --features shake256x4` fails with E0433 — the call sites flip to a type
+# that was never compiled. Guarded by tests/feature_forwarding.rs.
+shake256x4 = ["lib-q-fn-dsa-comm/shake256x4"]