v0.14.3 — Navigation enrichment: slug resolution and richer dropdowns

The v0.14.3 milestone delivers two coordinated navigation specs: SPEC-055 fixes nav slug ambiguity and the multiple-items-active-at-once symptom by moving slug resolution to build time and introducing multi-segment slugs as the first-class disambiguator; SPEC-054 extends the existing nav layouts so authors can compose Linear- / Vercel- / Stripe-style header dropdowns from the primitives they already know — layout="menubar" accepts arbitrary block content inside ## groups with a position-based intro / footer slot rule, layout="columns" gains a ----between-sections column-flow rule plus a headingless mode, and a new layout="strip" lands for compact secondary link rows. Plus a new core {% badge %} inline rune and a generalisation of auto=true description / icon enrichment to every layout.

SPEC-055 is foundational — it has to land first because the menubar enrichment is much cleaner once URL resolution is build-time and unambiguous. SPEC-054 layers on top, reusing the slug resolution path and the existing auto=true frontmatter enrichment machinery.

A patch milestone scoped to navigation polish. Builds on SPEC-046 (which defined the original layout attribute and deferred mega-style menus + per-item enrichment to future work).