test: add unit tests for RPC framing, SDK state machine, and server domain services

Add comprehensive tests across three layers:
- RPC framing: empty payloads, max boundary, truncated frames, multi-frame buffers,
  all status codes, all method ID ranges, payload-too-large for response/push
- SDK: event broadcast send/receive, multiple subscribers, clone preservation,
  conversation upsert, missing conversation, message ID roundtrip, member keys
- Server domain: auth session validation/expiry, channel creation/symmetry/validation,
  delivery peek/ack/sequence ordering/fetch-limited, key package upload/fetch/validation,
  hybrid key batch fetch, size boundary tests
- CI: MSRV (1.75) check job, macOS cross-platform build check

.github/workflows/ci.yml

@@ -113,6 +113,57 @@ jobs:
           name: coverage-report
           path: coverage/cobertura.xml
 
+  msrv:
+    name: MSRV Check
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install MSRV Rust (1.75)
+        uses: dtolnay/rust-action@1.75
+        with:
+          components: clippy
+
+      - name: Install capnp
+        run: sudo apt-get update && sudo apt-get install -y capnproto
+
+      - name: Cache cargo
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            target
+          key: ${{ runner.os }}-msrv-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-msrv-
+
+      - name: Check MSRV
+        run: cargo check --workspace
+
+  macos:
+    name: macOS Build Check
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Rust
+        uses: dtolnay/rust-action@stable
+
+      - name: Cache cargo
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            target
+          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-cargo-
+
+      - name: Check build
+        run: cargo check --workspace
+
   docker:
     name: Docker Build
     runs-on: ubuntu-latest