chore: changeset for book navigation (ADR-0046 §6)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: os-zhuang

.changeset/book-navigation-spine.md

@@ -0,0 +1,14 @@
+---
+"@objectstack/spec": minor
+"@objectstack/objectql": minor
+"@objectstack/rest": minor
+"@objectstack/runtime": patch
+---
+
+feat(book): documentation navigation as a `book` element — spine + derived membership (ADR-0046 §6)
+
+Adds the `book` metadata element: a navigation **spine** (ordered groups + `audience` + identity) whose membership is **derived** by rule (`include` glob/tag) plus optional per-doc `order`/`group`, never a central array. This keeps AI authoring create-and-forget (no central-array read-modify-write) and runtime overlay merge-safe (RFC 7396 treats arrays atomically).
+
+- `BookSchema` + `resolveBookTree()` derived-membership resolver + `defineBook()` + additive `doc.order`/`doc.group`.
+- Register `book` as a render-time metadata type (`allowOrgOverride: true`); wire it through the runtime type enumerations (PLURAL_TO_SINGULAR, engine registration, artifact field map, type-schema map).
+- REST `GET /meta/book/:name/tree` resolves the tree; read-layer `audience` gating (`public` ≡ anonymous; `org`/`{profile}` require sign-in).