feat: add e2e tests, perf benchmarks, and infrastructure improvements

- messages/bench_test.go: serialization benchmarks (msgpack map vs struct vs protobuf)
- clients/clients_test.go: HTTP client tests with pooling verification (20 tests)
- natsutil/natsutil_test.go: encode/decode roundtrip + binary data tests
- handler/handler_test.go: handler dispatch tests + benchmark
- config/config.go: live reload via fsnotify + RWMutex getter methods
- clients/clients.go: SharedTransport + sync.Pool buffer pooling
- messages/messages.go: typed structs with msgpack+json tags
- messages/proto/: protobuf schema + generated code

Benchmark baseline (ChatRequest roundtrip):
  MsgpackMap:    2949 ns/op, 36 allocs
  MsgpackStruct: 2030 ns/op, 13 allocs (31% faster, 64% fewer allocs)
  Protobuf:       793 ns/op,  8 allocs (73% faster, 78% fewer allocs)

go.sum

@@ -4,6 +4,8 @@ github.com/cespare/xxhash/v2 v2.3.0 h1:UL815xU9SqsFlibzuggzjXhog7bL6oX9BbNZnL2UF
 github.com/cespare/xxhash/v2 v2.3.0/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/fsnotify/fsnotify v1.9.0 h1:2Ml+OJNzbYCTzsxtv8vKSFD9PbJjmhYF14k/jKC7S9k=
+github.com/fsnotify/fsnotify v1.9.0/go.mod h1:8jBTzvmWwFyi3Pb8djgCCO5IBqzKJ/Jwo8TRcHyHii0=
 github.com/go-logr/logr v1.2.2/go.mod h1:jdQByPbusPIv2/zmleS9BjJVeZ6kBagPoEUsqbVz/1A=
 github.com/go-logr/logr v1.4.3 h1:CjnDlHq8ikf6E492q6eKboGOC0T8CDaOvkHCIg8idEI=
 github.com/go-logr/logr v1.4.3/go.mod h1:9T104GzyrTigFIr8wt5mBrctHMim0Nb2HLGrmQ40KvY=