Epic: Serialization and the single codec #443

Specs: 5-graph-trace-integrity.md ·
5.2-recovery-serialization.md
Plans: single-codec.md ·
structured-document-io.md

One canonical model; JSON, YAML and protobuf are renderings of it, and the same logical document produces
the same checksum through any of them. Format must never leak into identity.

Landed: op.SaveGraph / op.SaveTrace — the artifacts serialize themselves, format stated rather than
sniffed from a filename, checksum stamped by the writer because the reader refuses a document without one.

Remaining: document.Write's successor and its 18-19 config-shaped callers, the unlegislated cases
(non-integral floats, int64 past 2^53 through encoding/json, null-versus-absent), and cross-platform
document identity — nothing today proves a graph written on Windows loads on Linux with a matching
checksum.