Add Belgium as a third country toggle

[MaxGhenis]

Part of the populace-be epic PolicyEngine/populace#259. Depends on the BE release channel issue there (private HF repo + release contract artifacts must exist first).

What

Add Belgium as the third country in the dashboard, following the UK integration pattern:

• Env config: POPULACE_BE_HF_REPO / POPULACE_BE_HF_REVISION (private repo — token required, like the UK), plus the BE staging repo for the staging monitor.
• Country param plumbed through the API routes and data layer (frontend/lib/populace/latest-artifact.ts and staging-artifact.ts are already country-parameterized; extend the country union and toggle UI).
• Target-name decomposition for BE ledger naming (publisher prefixes like statbel.*, onss.*, bfp.*; geography facets = commune NIS / NUTS instead of FIPS/state — keep facets generic, no US-taxonomy assumptions like filing-status enums).
• Release overview, per-target browser, version compare, staging, and reform-validation views all work for ?country=be; reform view labels the BFP/EUROMOD-BE comparators the release manifest names.

Acceptance

• /populace?country=be renders the release summary and per-target browser from the private repo with a configured token.
• Country toggle shows US / UK / BE; no view crashes on BE's naming or geography dimensions.
• Data-layer tests cover BE name decomposition (extend latest-artifact.test.ts).