feat: add pg_replication module for reactive WAL-based cache invalidation

Implements a PostgreSQL logical replication module that streams WAL
changes, buffers them per XID for transaction atomicity, and publishes
CacheKey invalidation events to the existing framework broadcast channel.

Key design choices:
- Transaction atomicity: changes are buffered until COMMIT so the fanout
  always receives a consistent view. A 1000-row bulk-insert emits one
  table-level CacheKey, not 1000 row events.
- Native protocol: uses a self-contained raw TCP + postgres-protocol
  implementation for the replication connection (tokio-postgres 0.7 does
  not expose copy_both_simple publicly).
- Corrected pgoutput v1 parser: original maybe_sql_integration code
  incorrectly read XID from DML messages; DML messages carry no XID in
  proto v1 — only Begin does.
- Clean integration: publishes CacheKey::Channel events to store.events
  so the existing fanout task picks them up with zero changes to the
  fanout loop.
- New cx.subscribe(key) API on RenderContext for manually registering
  subscription keys (needed when queries go through PgPool, not cx.run).

New files:
  src/pg_replication/mod.rs       — PgReplicationListener, key helpers
  src/pg_replication/wal_parser.rs — pgoutput v1 binary protocol parser
  src/pg_replication/emitter.rs   — WalEmitter (tx-buffering + CacheKey emit)
  src/pg_replication/proto.rs     — raw TCP PG wire-protocol connection
  examples/pg_replication/main.rs — live message board example
  docker-compose.yml              — PG 16 container with wal_level=logical

Run the example:
  docker compose up -d
  cargo run --example pg_replication --features pg_replication

https://claude.ai/code/session_01SLGgXeqKV2o7KTmCaQZZPg

docker-compose.yml

@@ -0,0 +1,30 @@
+services:
+  postgres:
+    image: postgres:16-alpine
+    container_name: livevue_postgres
+    restart: unless-stopped
+    environment:
+      POSTGRES_USER: postgres
+      POSTGRES_PASSWORD: postgres
+      POSTGRES_DB: livevue
+    ports:
+      - "5432:5432"
+    volumes:
+      - postgres_data:/var/lib/postgresql/data
+    # Enable logical replication — required for the pg_replication example.
+    command:
+      - postgres
+      - -c
+      - wal_level=logical
+      - -c
+      - max_replication_slots=10
+      - -c
+      - max_wal_senders=10
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U postgres -d livevue"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+
+volumes:
+  postgres_data: