File-format feature flags: keep one bit permanently unassigned for 'unknown feature' tests, and pick prototype bits away from it

When a binary format uses incompat feature bitmasks (readers must reject unknown bits), the conformance suite usually has a reject vector that sets some 'unknown' bit. If a prototype feature later claims that same bit, any reader that understands the prototype silently breaks the reject test. Seen in practice: a proposed prototype gate bit was exactly the top bit the test vector used. Cheap rules: (1) reserve one bit in every feature word as never-assigned and document it as the test bit; (2) give prototypes a private bit elsewhere, keep their files internal and disposable, and regenerate them if the bit moves; (3) when the feature is proven (audited, benchmarked, reviewed), promote it to a regular standard bit with normative rules and its own accept/reject vectors, rather than carrying an 'experimental' range in the public spec. Also check whether a feature creates new cross-block dependencies (e.g. a shared compression prefix from the first block); those change random-access read cost and need measuring before promotion.