snapshotMeta_roundTripsAtPlausibilityCeiling

Mirrors RaftEngine.checkedRestoredSnapshotMeta (#1887): both halves of SnapshotMeta are admitted up to and including MAX_PLAUSIBLE, so both must survive a save/load cycle exactly.

The engine adopts lastIncludedTerm as state.snapshotTerm and lastIncludedIndex as state.snapshotIndex, which seeds the positional log math, the compaction floor, commitIndex, and the entries(snapshotIndex + 1) filter. §5.4.1 orders log positions by (term, index) lexicographically, so a value that drifts upward through a lossy column makes the node unbeatable in every election while carrying a log it cannot justify.

Why MAX_PLAUSIBLE - 1 and not just the ceiling. The ceiling is 2^60, a power of two and therefore exactly representable as an IEEE-754 Double — the adapter persisting a Long through a Double that this suite exists to catch round-trips it byte-for-byte. 2^60 - 1 needs 60 mantissa bits, so it is the value that actually probes precision. Both are asserted: a 32-bit column fails on either, a Double column only on MAX_PLAUSIBLE - 1.