phase 4: connectrpc on the gateway authenticated edge

Replace the native-gRPC server bootstrap with a single
`connectrpc.com/connect` HTTP/h2c listener. Connect-Go natively
serves Connect, gRPC, and gRPC-Web on the same port, so browsers can
now reach the authenticated surface without giving up the gRPC
framing native and desktop clients may use later. The decorator
stack (envelope → session → payload-hash → signature →
freshness/replay → rate-limit → routing/push) is reused unchanged
behind a small Connect → gRPC adapter and a `grpc.ServerStream`
shim around `*connect.ServerStream`.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

integration/go.sum

@@ -1,5 +1,7 @@
 buf.build/gen/go/bufbuild/protovalidate/protocolbuffers/go v1.36.11-20260209202127-80ab13bee0bf.1 h1:PMmTMyvHScV9Mn8wc6ASge9uRcHy0jtqPd+fM35LmsQ=
 buf.build/gen/go/bufbuild/protovalidate/protocolbuffers/go v1.36.11-20260209202127-80ab13bee0bf.1/go.mod h1:tvtbpgaVXZX4g6Pn+AnzFycuRK3MOz5HJfEGeEllXYM=
+connectrpc.com/connect v1.19.2 h1:McQ83FGdzL+t60peksi0gXC7MQ/iLKgLduAnThbM0mo=
+connectrpc.com/connect v1.19.2/go.mod h1:tN20fjdGlewnSFeZxLKb0xwIZ6ozc3OQs2hTXy4du9w=
 dario.cat/mergo v1.0.2 h1:85+piFYR1tMbRrLcDwR18y4UKJ3aH1Tbzi24VRW1TK8=
 dario.cat/mergo v1.0.2/go.mod h1:E/hbnu0NxMFBjpMIE34DRGLWqDy0g5FuKDhCb31ngxA=
 github.com/AdaLogics/go-fuzz-headers v0.0.0-20240806141605-e8a1dd7889d6 h1:He8afgbRMd7mFxO99hRNu+6tazq8nFF9lIwo9JFroBk=