Add X25519MLKEM768 post-quantum hybrid key exchange

Implement the X25519MLKEM768 hybrid key exchange group per
draft-ietf-tls-ecdhe-mlkem-00, combining ML-KEM-768 (FIPS 203) with
X25519 for post-quantum/classical hybrid key agreement.

- New mlkem feature gating X25519MLKEM768 via boring's mlkem module
- fips feature now implies mlkem so PQ is always available in FIPS mode
- X25519MLKEM768 is the preferred (first) group in both FIPS and
  non-FIPS configurations when mlkem is enabled
- Uses boring::mlkem for ML-KEM-768 and direct X25519 FFI for the
  classical component (no SPKI overhead)
- Overrides start_and_complete() for server-side KEM encapsulation
- Wire format: ML-KEM component first in all encodings (client share
  1216 bytes, server share 1120 bytes, shared secret 64 bytes)
- TLS 1.3 only (usable_for_version rejects TLS 1.2)
- Unit tests: hybrid round-trip, invalid share rejection, version
  and FIPS flag checks
- E2E tests: self-to-self PQ TLS handshake, FIPS group assertions
- Cloudflare interop tests (ignored): verify kex=X25519MLKEM768 via
  /cdn-cgi/trace for TLS 1.3, verify classical fallback for TLS 1.2
- Update README with PQ section, feature docs, and FIPS KX updates

.github/workflows/ci.yml

@@ -63,6 +63,44 @@ jobs:
     - name: Build (logging,tls12)
       run: make build FEATURES="logging,tls12"
 
+  test-mlkem:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v3
+    - name: Install dependencies
+      run: sudo apt-get install -y cmake clang
+    - name: Lint (mlkem)
+      run: make lint FEATURES="mlkem"
+    - name: Test (mlkem)
+      run: make test FEATURES="mlkem"
+    - name: Build (mlkem)
+      run: make build FEATURES="mlkem"
+
+  test-mlkem-tls12:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v3
+    - name: Install dependencies
+      run: sudo apt-get install -y cmake clang
+    - name: Lint (mlkem,tls12)
+      run: make lint FEATURES="mlkem,tls12"
+    - name: Test (mlkem,tls12)
+      run: make test FEATURES="mlkem,tls12"
+    - name: Build (mlkem,tls12)
+      run: make build FEATURES="mlkem,tls12"
+
+  test-fips:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v3
+    - name: Install dependencies
+      run: sudo apt-get install -y cmake clang
+    - name: Test (fips)
+      run: cargo test -p boring-rustls-provider --all-targets --features fips
+
   check-fips:
     runs-on: ubuntu-latest