feat(napi): add RwSite.listPages() enumerating pages with titles

[yumike]

Closes #576.

Summary

Adds a reload-aware napi method that enumerates every page in a site, each with its title and its (sectionRef, subpath) key — the same pair the comment system uses as a page's document_id:

RwSite.listPages(): Promise<Array<{ sectionRef: string; subpath: string; title: string }>>

Motivation

The Backstage doc-comment inbox (rwdocs/backstage-plugins) wants to show a human page title instead of a raw path slug, and caches titles for all pages per site (rebuilt on the ownership-rebuild schedule, when each site is already loaded in memory). It needs to enumerate every page with its title in one cheap pass. The existing surface can't: listSections() enumerates sections (no title), and renderPage() does a full render (Kroki, etc.) — far too costly per page. Resolving titles at serve time is also too expensive (the S3-mode Hub LRU-caches ~20 sites, so a broad inbox would force repeated site loads).

Implementation

A direct three-layer mirror of the existing listSections():

• SiteState::list_pages() — walks SiteState.pages, computes each page's (section_ref, subpath) via the existing section_location seam, reads Page.title, sorts by (section_ref, subpath). New public PageEntry struct.
• Site::list_pages() — reload_if_needed()? then delegates (mirrors Site::list_sections).
• napi listPages() — spawn_blocking wrapper; new PageEntryResponse; regenerated index.d.ts.

Naming

The enumerated entity is a Page (matches Page / renderPage / SiteState.pages and parallels the sibling listSections/Section), and the set includes virtual directory pages that aren't comment "documents" — so listPages/PageEntry is the accurate, codebase-consistent name. The (sectionRef, subpath) key is still a page's comment document_id. (The issue proposed listDocuments; renamed to listPages after review.)

Scope

Every page is included — the root page (empty subpath) and virtual directory pages — so any comment ever keyed to one still resolves a title. Fields are intentionally minimal (sectionRef, subpath, title); path/ancestors/order are omitted and can be added additively later.

Testing

• 5 SiteState::list_pages unit tests (root, nested-in-section, outside-any-section, virtual page, page_path_for round-trip, sort order) + 1 Site::list_pages integration test.
• Full rw-site suite green (150 unit + 4 doctests); rw-napi builds clean; index.d.ts regenerated to listPages(): Promise<Array<PageEntryResponse>>.

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