The 246-byte all-absent Edge vector (SM-10) existed only on the Rust
side (selection_wire_parity.rs's CSHARP_EDGE_HEX). The all-absent
header gets incidental cover because it is nested inside the 376-byte
envelope test; the edge has no such cover, so a C# edge encoder that
resumed omitting a null payload/payloadEncoding would still pass every
other test here - exactly the asymmetry R-AP existed to kill.
Adds EdgeAllAbsentMatchesTheConstantCommittedIntoTheRustTest, asserting
the same constant against MessagePackSerializer.Serialize. Flipping one
hex digit of RustEdgeAllAbsentHex fails the new test only, confirming
it is not vacuous.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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