kill protobuf from default build: OTLP exporter now behind -tags otlp

Pre: importing luxfi/trace pulled go.opentelemetry.io/proto/otlp/* which
transitively requires google.golang.org/protobuf — even when callers
used trace.Noop. Any pkg in the workspace that touched luxfi/vm/api or
luxfi/evm/api inherited 11MB of OTLP protobuf code it never executed.

Post: ZERO OTLP / ZERO protobuf in the default dep graph.

Files:
  - exporter_config.go (new, no build tag): ExporterConfig moved out
    of exporter_zap.go so every build configuration exposes the
    same Config type to callers.
  - exporter_noop.go (new, //go:build !grpc && !otlp): default-build
    newExporter returns errNoOTLPExporter — callers must set Type=Disabled
    (returns trace.Noop, no exporter constructed) or rebuild with
    -tags otlp.
  - exporter_zap.go (//go:build !grpc && otlp): existing OTLP/HTTP+protobuf
    exporter, now opt-in. Renamed conceptually but file name kept to
    avoid breaking external scripts that grep for it.

Verification:
    go list -deps ./... | grep -E 'protobuf|otlp'   → empty
    go build ./...                                  → ok
    go build -tags otlp ./...                       → ok

Per the project rule: ZAP internal, ZIP edge. The OTLP exporter is
strictly protobuf-on-the-wire — it stays behind -tags otlp where any
consumer can opt in.

exporter_config.go

@@ -0,0 +1,21 @@
+// Copyright (C) 2019-2025, Lux Industries Inc. All rights reserved.
+// See the file LICENSE for licensing terms.
+
+// ExporterConfig lives outside the tagged exporter files because every
+// build configuration (default, -tags otlp, -tags grpc) needs to expose
+// the same configuration type to callers.
+
+package trace
+
+type ExporterConfig struct {
+	Type ExporterType `json:"type"`
+
+	// Endpoint to send traces to. If empty, the default endpoint will be used.
+	Endpoint string `json:"endpoint"`
+
+	// Headers to send with traces
+	Headers map[string]string `json:"headers"`
+
+	// If true, don't use TLS
+	Insecure bool `json:"insecure"`
+}

exporter_noop.go

@@ -0,0 +1,34 @@
+//go:build !grpc && !otlp
+
+// Copyright (C) 2019-2025, Lux Industries Inc. All rights reserved.
+// See the file LICENSE for licensing terms.
+
+// Default build: no OTLP/protobuf exporter. Callers who want OTLP export
+// must opt in with -tags otlp (HTTP+protobuf) or -tags grpc (legacy gRPC).
+//
+// With this file compiled, trace.New(Config{Type: Disabled}) returns
+// trace.Noop and no go.opentelemetry.io/proto/otlp/* or
+// google.golang.org/protobuf import is pulled into the binary.
+
+package trace
+
+import (
+	"errors"
+
+	sdktrace "go.opentelemetry.io/otel/sdk/trace"
+)
+
+// errNoOTLPExporter is returned when a caller asks for a real exporter on
+// a default (otlp-untagged) build. Rebuild with -tags otlp to enable
+// OTLP/HTTP export, or set Config.ExporterConfig.Type = Disabled to use
+// the noop tracer.
+var errNoOTLPExporter = errors.New(
+	"trace: OTLP exporter not compiled (rebuild with -tags otlp or use ExporterConfig.Type=Disabled)",
+)
+
+// newExporter — default build returns an error. Callers should set
+// Type=Disabled (which short-circuits before this is reached in
+// trace.New) or rebuild with -tags otlp.
+func newExporter(_ ExporterConfig) (sdktrace.SpanExporter, error) {
+	return nil, errNoOTLPExporter
+}

exporter_zap.go

@@ -1,8 +1,13 @@
-//go:build !grpc
+//go:build !grpc && otlp
 
 // Copyright (C) 2019-2025, Lux Industries Inc. All rights reserved.
 // See the file LICENSE for licensing terms.
 
+// OTLP/HTTP+protobuf exporter. Compiled only with -tags otlp because it
+// pulls go.opentelemetry.io/proto/otlp/* — which transitively requires
+// google.golang.org/protobuf. Default builds use exporter_noop.go and
+// have zero protobuf in the dep graph.
+
 package trace
 
 import (
@@ -27,19 +32,6 @@ import (
 
 const tracerProviderExportCreationTimeout = 5 * time.Second
 
-type ExporterConfig struct {
-	Type ExporterType `json:"type"`
-
-	// Endpoint to send traces to. If empty, the default endpoint will be used.
-	Endpoint string `json:"endpoint"`
-
-	// Headers to send with traces
-	Headers map[string]string `json:"headers"`
-
-	// If true, don't use TLS
-	Insecure bool `json:"insecure"`
-}
-
 func newExporter(config ExporterConfig) (sdktrace.SpanExporter, error) {
 	var client otlptrace.Client
 	switch config.Type {