Constant SAVE_FORMAT_VERSION 

pub const SAVE_FORMAT_VERSION: u32 = 1;

The canonical-JSON save format version. Bump the (single) integer on any wire-incompatible change to the envelope the loader consumes. The loader reads a top-level hyperlark_format_version and rejects a value greater than this (a newer save cannot be trusted to parse under an older build); an absent header is accepted — the corpus fixtures and the loader’s own input format predate the header and are, by construction, the current pin’s shape.

A key an older reader would silently IGNORE counts as wire-incompatible: hyperlark_declared_terminals sits at version 1 only because beta.1 is the first release, so no published reader predates it. The next such key bumps this, stamped only on the saves that carry it — a reader must refuse exactly what it would mis-load, not every save from a newer build.