Constructor-built immutable recipes, replay-proof crafting, an append-only audit journal, and canonical JSON saves.

NEXUS Crafting is a deterministic crafting engine for Unity. Recipes, journal entries and snapshots are immutable: they are built through constructors and can never be edited afterwards — assignment to their properties is a compile-time error, so entire bug classes are structurally impossible.

- Four-state-safe engine: recipe lookup, discovery locks, station tags, atomic input consumption, replay rejection — every outcome is a typed result with a precise detail string.

- Replay-proof idempotency keys: pass your own key (quest step, order id) or let the engine mint one. Re-submitting a key costs nothing — inventory, journal and result all stay untouched.

- Append-only audit journal: strict 1-based batches, forever-unique keys, UTC timestamps, read-only exposure. "What did this player do?" is always answerable.

- Canonical JSON snapshots: NCRAFT v1 envelope, fixed key order, byte-stable output. Strict decode rejects wrong magic, wrong versions, duplicate keys, non-ascending batches and zero amounts — corrupted saves fail at load time with the offending byte offset.

- Authoring that validates itself: recipe libraries report empty ids, zero amounts, duplicates and self-feeding loops; the project-wide validator catches cross-library id clashes.

- Editor tools: Recipe Browser, live Journal Inspector, one-click project validation, showcase opener — all under Tools > NEXUS Crafting.

- Zero class-level static fields and zero singletons: safe with Enter Play Mode Options even with domain reload disabled.

- Optional companions, never dependencies: NEXUS Inventory adapts onto a four-method interface; snapshots drop into NEXUS Persistence as plain text.

Zero shaders, zero materials, zero rendering code. Unity 2021.3 LTS or newer, all render pipelines, all renderers. 55 EditMode tests included — run them in Test Runner right after import, no Play Mode needed.