Feature flags for the v2 compact wire format.
All bits are reserved for future use. Unknown flags must be ignored
by the decoder (forward compatibility).
Magic + version prefix for the compact binary wire format.
FB1 = Fib Binary v1. Any decoder that sees a different
magic should reject the payload as corrupt.
Magic + version for the v2 compact wire format with feature flags.
FB2 = Fib Binary v2. Adds a 1-byte feature flags field
after the version byte, enabling forward-compatible extensions
(alternative norm encodings, sparse indices, etc.).