refactor(demos): framing test helpers are split / join, like the pipes

[kryptt]

Follow-up to #133. The gRPC framing pipes were renamed splitFrames / joinFrames there, but GrpcFramesSpec's helpers kept the old names, decode and framed. In the demo's benchmarks decode means decoding a whole Issue (the baseline judge), so a framing helper with that name read as the same thing.

• GrpcFramesSpec:
  • decode → split, framed → join.
  • join moves to the companion, next to pieces. PipelineSpec now frames its upstream with it, the way StreamBench does with joinFrames.
• GrpcFrames.Malformed: its doc names splitFrames instead of "this decoder".
• Cookbook, "Allocation-free stream triage demo":
  • the GrpcFramesSpec row now says messages are joined into frames and split back whole (it said "decode back whole");
  • the comments in the framing code block line up again.

The benchmarks keep decode for the full-decode judge, which is what the recipe's benchmark tables call it.

Checked locally: demoGrpcFilter/test passes (35 tests), demoGrpcFilterBench/Jmh/compile succeeds, and the scalafmt and scalafix checks are clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01JG5LJsPA524HKbSMvjStrv

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