Brummel
cb558519b0
Iteration within the C24 topology-as-data milestone (#156). Codifies the blueprint format's additive forward-compatibility under one discipline: Tier-1 (additive-optional) = serde-default silent-ignore, no version bump, C1 default-equals-prior-behaviour; Tier-2 (must-understand: new node type, edge semantics, structural-axis kind) = bump format_version so an old reader refuses cleanly rather than building a different graph. Most of #156 already shipped with #155 (cycle 0087): the format_version envelope, omit-defaults canonical form, and both Tier-2 load-path refusals (LoadError::UnsupportedVersion / UnknownNodeType) are present and green. The genuine remainder is one unproven property — forward-tolerance of an unknown optional field on the current loader — plus codifying the discipline in the LoadError doc and the C24 ledger entry. Not a new mechanism. Derived fork recorded on #156: the per-section required-flag mechanism (PNG critical-vs-ancillary) is NOT built — the version envelope already gives the must-understand refusal at version granularity, there is no current Tier-2 section to validate a finer scheme against, and C1 holds either way. Boss-signed on a grounding-check PASS (all current-behaviour claims ratified by named green tests; the new Tier-1 pin correctly absent — this cycle's deliverable). refs #156