perf(client-mod): candidate WebSocket codec, codecBenchmark and decision rule

TCP_NODELAY on ws/wss; ClientFrameEncoder (one message per batch) and
ClientFrameDecoder (retained slices) with tests. The decision rule is
recorded before the measured runs; the verdict follows in a later commit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

testing/benchmarks/results/2026-10-23-client-websocket-codec.md

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+# Client mod: own WebSocket codec instead of Netty 4.0.10's — 2026-10-23
+
+## Decision rule (set before the measured runs)
+
+Encoder (`ClientFrameEncoder`) is kept only if all hold:
+- median messages per 1000 packets is at most a third of the base median;
+- median event-loop CPU ns/packet is not above the base median;
+- a lone packet still leaves in the event-loop task right after its write (`ClientFrameEncoderTest.lonePacketLeavesInTheTaskRightAfterItsWrite`).
+
+Decoder (`ClientFrameDecoder`) is kept only if, in both decode views (codec only, and codec plus vanilla's splitter), the median event-loop CPU ns/byte and the median allocated bytes/byte are each at least 25% below the base median, and the candidate's min–max range does not overlap the base's for either metric.
+
+If the decoder misses and the encoder meets its rule, only the encoder is kept.
+
+Order: six JVMs, base/candidate/candidate/base/base/candidate (the starting variant alternates per pair), each with one warm-up phase and three measured phases per mode. Results are the median and [min–max] of the 9 measured phases per variant.