Two-task RED→GREEN plan for spec 0033. Task 1 (RED, test-only): flip the engine
canonical-form golden + the CLI sweep golden to the serde array-of-pairs/.0-float
shape, add the to_json_equals_serde_disk_shape pin test — all fail against the
current hand-rolled to_json. Task 2 (GREEN, atomic): promote serde_json dev-dep ->
normal dep, swap to_json's body to serde_json::to_string, delete the now-unused
json_str helper, refresh the method + module-header doc comments — one build+clippy
gate flips every Task-1 assertion green. graph_model.rs:30 + INDEX.md:729 stale
cross-refs deferred to cycle-close audit.
refs #54
Brummel
e2056b6436