docs(adr-0046): P3 increment — book navigation as a spine with derived membership (#1865)

Adds §6 to ADR-0046 specifying the P3 navigation model. A `book` is a metadata
element that stores only an ordered spine (groups); membership is derived by
rule (`include` glob/tag) plus optional per-doc `order`/`group`, never a central
array. This is chosen so AI authoring stays create-and-forget (no central-array
read-modify-write, §6.2.1) and runtime overlay stays merge-safe under RFC 7396.

Also specified: the implicit per-package book (packageId), `audience` access
(org / public≡guest / {profile}) enforced at the read layer, and a host-based
anonymous "library" portal reusing apps/docs (Fumadocs + per-host ISR) rather
than greenfield SSR. No `meta.json`: `book` is metadata, authored as *.book.ts.

Co-authored-by: Jack Zhuang <277994282+os-zhuang@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 3 people