Keep malformed or missing CDP opcodes and payload facts unknown, retain observed zero and empty payloads, and avoid false capture differences while preserving independently known facts.
Validation: 59 source tests; compiled public CLI and stdio MCP fake-CDP acceptance; check:fast; docs:check; changed-file formatting. Real Chromium malformed-event behavior was not tested.
BREAKING CHANGE: WebSocket event opcode is nullable and numeric values are restricted to the four-bit protocol range.
Fixes#5666