otap: real plugin integration + Arrow IPC network transport

[zzylol]

Stacked on #5 (feat/schema-dictionary-record-codec), which implemented the SeriesDictionary/SeriesDictionaryDecoder codec but only proved it end-to-end via direct calls in one process, in-memory — not through the actual OTAP plugin lifecycle, and not across a real serialize/transmit/deserialize hop. This PR closes both gaps, per review feedback on that PR's demo.

What changed

• otap::wire (new): Arrow-IPC serializes a SketchStreamBatch's four RecordBatches into a length-prefixed frame, plus async send_stream_batch/recv_stream_batch over a TcpStream. Each sub-batch is its own self-contained IPC stream (schema + one record batch + EOS); recv_stream_batch distinguishes a clean EOF between frames from a truncated one mid-frame.
• AsapSketchesPlugin::start_from_envelopes (new): the receiver-role counterpart to the existing producer-role start(). Consumes Stream<Item = SketchStreamBatch> instead of Stream<Item = OtapMetricRecords>, decodes via a persistent SeriesDictionaryDecoder, and routes reconstructed envelopes through Precompute::observe_envelope (merge, never expand to samples) — reusing the same ticker/control-task/graceful-drain machinery as the producer role via a new shared spawn_lifecycle helper. A receiver configured with transmit_sketch = false naturally re-emits query-mode (quantile) estimates instead of sketch bytes through its own emit channel, so a chain of AsapSketchesPlugins can compose without any new machinery.
• examples/sketch_producer_node.rs + sketch_receiver_node.rs (new): two separate binaries — real AsapSketchesPlugin producer and receiver roles, connected over a real TCP socket via otap::wire, not the in-process mpsc channel sketch_pipeline_demo.rs uses. The producer feeds a real OTAP-shaped input stream (records::flatten + decode_batch, not a direct observe() call) and lets the plugin's actual Wakeup-style Tokio ticker close windows on its own wall-clock schedule.

examples/sketch_pipeline_demo.rs (from #5) is unchanged and still works — it remains the fast, single-process way to see the codec logic; these two binaries are the higher-fidelity version.

Testing

• cargo test --features otap: 156/156 passing (7 new: otap::wire's IPC round-trip + real TCP loopback tests, AsapSketchesPlugin's receiver-role smoke test and full producer→receiver chain test).
• cargo fmt --check / cargo clippy --features otap --tests --examples: clean.
• Ran both new binaries together end to end: producer emits 5 windows (window 0 carries SCHEMA+DICTIONARY+LABELS, windows 1-4 carry RECORD only — dictionary economics holding over the real socket), receiver receives and Arrow-IPC-decodes all 5, merges them, and prints a correct p99 gauge.

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