Fossil-in-Zig, ported from the four RFC drafts (docs/rfcs/). Layout mirrors the RFC layering: delta -> artifact -> repostate -> sync. delta int codec + checksum and artifact naming/Z-card implemented with RFC-derived test vectors; everything else stubbed against its section. Builds clean; zig build test green. See docs/HANDOFF.md. check-in: 0ae7fb2622 user: dmestas tags: trunk