Add the module navigation contract: manifest v9, module routes, return-to-origin

Stage 1 PR 7 (ADR 0063). A module becomes a first-class part of the Dashboard.

- Manifest v9 = v8 plus optional `navigation`: a Lucide icon from the curated
  MODULE_NAVIGATION_ICONS set, and up to 12 primary pages (stable id, safe
  label, kebab-case path relative to the module root; no traversal, schemes,
  queries, duplicates, or nesting). Validated before activation.
- Version 8 stays accepted beside version 9, each read strictly as itself
  (discriminated union); no released manifest is rewritten. The template
  moves to v9; the released Signage 0.1.5 manifest is a test fixture.
- Real module routes: /<id> is the landing view, /<id>/<page> a declared
  page, anything beneath a page a module-owned detail route; anything else
  is not-found and mounts nothing. api/assets/display are never pages.
- Sidebar: declared icons, pages nested under an open module, one
  aria-current, collapsed and drawer behaviour; v8 modules unchanged.
- module-ui-sdk: ModuleUiHostContext.navigation (location, subscribe,
  navigate, link, settings), refusing anything outside the module's routes;
  undefined on the display host. Mount ABI unchanged.
- Return-to-origin: one `?return=` carrier on Settings URLs, validated by
  parseReturnTarget and against running modules' declared routes; Settings
  Back returns to exactly the originating module page, across refreshes.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/architecture/decisions/README.md

@@ -71,3 +71,4 @@ ADRs capture durable choices that affect more than one package or phase.
 - [0060: Light, Dark, and System appearance; reduced motion; and one radius scale](0060-light-dark-system-theme-and-motion.md) — Accepted
 - [0061: Settings is one host-owned hierarchy; module lifecycle is Discover, and a module's settings live at `/settings/<module-id>`](0061-settings-hierarchy-and-module-settings-routes.md) — Accepted
 - [0062: The account area, and appearance preferences that belong to the account](0062-account-area-and-synced-appearance-preferences.md) — Accepted
+- [0063: Module navigation — manifest v9 icons and pages, real module routes, and return-to-origin](0063-module-navigation-contract-manifest-v9.md) — Accepted