pub const DOC_FLAGS_MASK: u8 = _; // 3u8Expand description
Mask of currently-defined flag bits. Bits set outside this mask
are reserved for future protocol versions; today’s encoder
rejects them with MessageError::InvalidFlags so that legacy
frames can’t enable not-yet-implemented behaviors. Round-2 of
peat-lite#26 added this reservation contract.
Adding a new flag: extend the mask, add the const, AND wire
the encode/decode handling for the new behavior. The
reserved_flag_bits_are_rejected test will start failing for the
new bit (since it’s no longer reserved), forcing the contributor
to update tests, which is the integration prompt that the
behavior wiring is also incomplete. There is no static-analysis
forcing function, so the test loop is the canary.