feat(maps): dual-ring markers with 5-tier color scale

Replace flat circle markers with nested dual-ring structure encoding
two scores per marker (core = primary, ring = secondary). Upgrade
from 3-tier to 5-tier colorblind-safe scale. Add pulse animation for
high-opportunity markers (>=75). Extract shared PNMarkers module from
3 duplicated implementations.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -6,6 +6,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ## [Unreleased]
 
+### Changed
+- **Dual-ring map markers** — map markers now encode two scores visually: inner core = primary score, outer ring = secondary score. Markets hub and country overview show Market Score (core) + Opportunity Score (ring). Opportunity map shows Opportunity Score (core) + Market Score (ring). City venue maps unchanged (navy dots). Color scale upgraded from 3-tier (green/amber/red) to 5-tier (deep green ≥80, teal ≥60, amber ≥40, orange-red ≥20, red <20) with distinct luminance at each tier for colorblind safety. Markers < 18px fall back to single-layer (no ring). Muted markers (cities without articles) show dashed ring outline. Highlighted markers (user's geo city) get blue outer glow. Opportunity map markers with score ≥75 pulse gently to highlight top investment targets. Tooltip lines now have inline color dots matching marker layers. All map scripts share a single `map-markers.js` module (`PNMarkers.scoreColor` + `PNMarkers.makeIcon`), replacing 3 duplicated implementations.
+
 ### Added
 - **Geo headers on city/region hubs** — Cloudflare geo headers (`CF-IPCountry`, `CF-IPCity`) now used across location-based pages. Opportunity map pre-selects and auto-loads the user's country. Country overview maps highlight the user's city with a blue ring (best-effort CF-IPCity name match). `window.__GEO` JS global injected via `base.html` for client-side map code.