mvp phase ε.1.5: sketchotap + sketchtelegraf Mode 3 emitters

[zzylol]

Summary

Phase ε.1.5 — re-dispatch the Phase ε.1 three-mode emit path across all three edge runtimes. Phase ε.1 (#319) shipped BindMode + WireCostTable + SketchExpr::RawAtEdgePrometheusArchive + the emit_edge_yaml Mode 3 branch for the OTel-collector runtime; this PR mirrors that for sketchotap (otap-dataflow Rust) and sketchtelegraf.

• New controller/src/config/stage_config_otap.rs — emit_otap_dag_yaml produces an otap-dataflow DAG (version: otel_dataflow/v1), receiver:otlp → optional processor:asap_sketches → exporter:otlp_grpc to gateway (Modes 1/2) or exporter:otlp_http to Prometheus's /api/v1/otlp/v1/metrics (Mode 3).
• New controller/src/config/stage_config_telegraf.rs — emit_telegraf_toml produces [[inputs.opentelemetry]] → optional [[processors.allsketches]] → [[outputs.opentelemetry]] to gateway (Modes 1/2) or [[outputs.http]] with data_format = "prometheusremotewrite" to Prometheus's /api/v1/write (Mode 3). Telegraf's outputs.opentelemetry is gRPC-only with no protocol = "http/protobuf" field; remote-write lands in the same Prometheus TSDB so the storage outcome is identical (only the wire framing differs).
• New AgentRuntime enum + emit_for_runtime dispatcher in controller/src/config/mod.rs. Reads X-Agent-Runtime OpAMP header (sketchcollector / sketchotap / sketchtelegraf); defaults to Sketchcollector when absent so legacy agents keep working. AgentRuntime did NOT exist on main — this is a new enum, parallel to the existing opamp::AgentRole.
• 16 new unit tests covering each runtime's three modes + the dispatcher; OTAP DAGs round-trip through a serde_yaml stub, Telegraf TOML through a purpose-built minimal validator that catches malformed table headers / unbalanced quotes.
• docs/control-plane-design.md documents the per-runtime emit paths.

Test plan

• cargo test --release -p controller — 549 passed, 10 pre-existing failures unchanged
• Mode 1 / Mode 2 / Mode 3 snapshot tests for each new runtime emitter
• Mode 3 outputs are parseable: OTAP via serde_yaml::from_str::<OtapDagStub>, Telegraf via the in-tree minimal TOML validator
• emit_for_runtime(Sketchcollector, ...) is identical to direct emit_edge_yaml (regression guard)

Mode 3 exporter shapes (one-line per runtime)

• sketchcollector (existing): exporters.otlphttp/prometheus.metrics_endpoint = "${ASAP_PROMETHEUS_OTLP_URL:-http://prometheus:9090/api/v1/otlp/v1/metrics}"
• sketchotap (new): node exporter of type exporter:otlp_http with metrics_endpoint: http://prometheus:9090/api/v1/otlp/v1/metrics
• sketchtelegraf (new): [[outputs.http]] url = "http://prometheus:9090/api/v1/write" + data_format = "prometheusremotewrite" (Telegraf-specific deviation — see PR body)

Plugin compatibility notes for Phase 3.3

• Telegraf outputs.opentelemetry plugin in our shipped tree (telegraf/plugins/outputs/opentelemetry/opentelemetry.go) is gRPC-only with no HTTP/protobuf option. Phase 3.3 demo runs that exercise sketchtelegraf Mode 3 must point Prometheus's --web.enable-remote-write-receiver flag (or use the dedicated remote-write endpoint) — Prometheus accepts both OTLP HTTP at /api/v1/otlp/v1/metrics and remote-write at /api/v1/write and stores them in the same TSDB.
• The processor:asap_sketches URN in the emitted OTAP DAG is the contract Phase 3.3's sketchotap binary must register against (via linkme::distributed_slice(OTAP_PROCESSOR_FACTORIES)); the plugin source lives in otap-patch/plugins/asap_sketches/ per docs/design-asap-otap-rust-integration.md.

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