[Copilot]

When MAX_DATA_LEN is overridden by a consumer, it must be at least as large as ControlChannelPayload (16 bytes) or channel 0 send/receive will overflow the payload buffer. Previously there was no enforcement of this constraint.

Changes

• src/SerialMuxProtCommon.hpp: Added static_assert after the ControlChannelPayload typedef to enforce the minimum size at compile time:

static_assert(MAX_DATA_LEN >= sizeof(ControlChannelPayload),
              "MAX_DATA_LEN must be >= sizeof(ControlChannelPayload) to avoid buffer overflows on control channel.");

Defining MAX_DATA_LEN below 16 now produces a clear compiler error indicating the failing size comparison, e.g. the comparison reduces to '(4 >= 16)'.

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