Plan for changes to CDP monitor

[archandatta]

Note

Low Risk
Documentation-only change that proposes a future refactor; it does not modify runtime behavior or data handling in this PR.

Overview
Adds plans/cdp-decode.md, a design plan to refactor cdpmonitor into a two-layer CDP decode: protocol-faithful Layer-1 structs (retaining all PDL fields, with complex sub-objects as json.RawMessage) and a Layer-2 projection that maps typed inputs to monitor events.

The doc also outlines intended file/module changes and a proposed golden round-trip test strategy to ensure decoded JSON frames preserve all fields for handled CDP events.

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[rgarcia]

i think you want to compare normalized full JSON after decode/re-marshal and not just top level keys, to get a full structurual round trip check

you could also consider some light codegen around the json schema in json/js_protocol.json in the devtools repo