Add connector, response policy, telemetry emitter and gateway

Completes the request path for FLUID-WP-0003 T01-T03 and T05-T07. The
gateway resolves a revision, routes to the adapter process, and records
what happened, without ever depending on the control plane to serve.

Three behaviours carry tests because the architecture rests on them:
Emit never blocks against a stalled sink (Blueprint 34.2), the gateway
keeps serving after control-plane loss (invariant 2), and backend
internals do not leak into error responses (5.7).

The connector retries only idempotent methods, so a slow adapter cannot
cause a hall-of-helix entry to be published twice, and breakers are
per-revision so a broken candidate does not take the stable revision
down with it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014KmVxhJ35tCo7rE7UnLwWu

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 1116572@bnt-lap001
Assistant-Session: 8ba9bb93-a72a-4883-b189-2499cce5c400

internal/runtime/response.go

@@ -0,0 +1,91 @@
+package runtime
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/tegwick/fluid-core/internal/contract"
+)
+
+// ErrorKind classifies a response the interface produced rather than the
+// backend. These map onto the telemetry error classes so that a response and
+// the evidence it generates cannot disagree.
+type ErrorKind string
+
+const (
+	ErrorValidation    ErrorKind = "validation"
+	ErrorUnknownPath   ErrorKind = "unknown_path"
+	ErrorUnsupported   ErrorKind = "unsupported_parameter"
+	ErrorAuthorization ErrorKind = "authorization"
+	ErrorUnavailable   ErrorKind = "backend_failure"
+	ErrorTimeout       ErrorKind = "timeout"
+	ErrorPolicy        ErrorKind = "policy_rejection"
+	ErrorNoCapability  ErrorKind = "missing_capability"
+)
+
+// ErrorBody is the interface's error representation.
+//
+// ArchitectureBlueprint.md section 5.7: errors should carry enough structured
+// information to improve observability without leaking backend detail, and may
+// include a correlation reference that lets downstream analysis link the
+// response a consumer saw to the pressure it generated.
+type ErrorBody struct {
+	Kind ErrorKind `json:"kind"`
+	// Message is written for the consumer and must stay free of backend
+	// internals: hostnames, stack traces, driver errors, upstream payloads.
+	Message string `json:"message"`
+	// Correlation is the reference a consumer can quote back, and the key that
+	// ties this response to its telemetry.
+	Correlation string `json:"correlation"`
+	// Revision tells the consumer which contract answered. Without it a client
+	// debugging an unexpected response has no way to know what it was talking to.
+	Revision contract.RevisionID `json:"revision,omitempty"`
+	// Field names the offending input for validation failures.
+	Field string `json:"field,omitempty"`
+	// Feedback points at the explicit-feedback endpoint. Turning a dead end into
+	// an invitation is the cheapest pressure signal the interface can collect.
+	Feedback string `json:"feedback,omitempty"`
+}
+
+// statusFor maps an error kind to its HTTP status.
+var statusFor = map[ErrorKind]int{
+	ErrorValidation:    http.StatusBadRequest,
+	ErrorUnknownPath:   http.StatusNotFound,
+	ErrorUnsupported:   http.StatusBadRequest,
+	ErrorAuthorization: http.StatusForbidden,
+	ErrorUnavailable:   http.StatusBadGateway,
+	ErrorTimeout:       http.StatusGatewayTimeout,
+	ErrorPolicy:        http.StatusForbidden,
+	ErrorNoCapability:  http.StatusNotImplemented,
+}
+
+// ResponsePolicy renders interface errors consistently.
+type ResponsePolicy struct {
+	// FeedbackPath, when set, is advertised on every error.
+	FeedbackPath string
+}
+
+// WriteError renders an error response.
+func (p ResponsePolicy) WriteError(w http.ResponseWriter, kind ErrorKind, correlation, message string, rev contract.RevisionID, field string) {
+	status, ok := statusFor[kind]
+	if !ok {
+		status = http.StatusInternalServerError
+	}
+
+	body := ErrorBody{
+		Kind:        kind,
+		Message:     message,
+		Correlation: correlation,
+		Revision:    rev,
+		Field:       field,
+		Feedback:    p.FeedbackPath,
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	w.Header().Set("X-FLUID-Correlation", correlation)
+	if rev != "" {
+		w.Header().Set("X-FLUID-Revision", string(rev))
+	}
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(body)
+}