feat: river rendering improvements — width classes and bridge overlays

[nikolareljin]

Summary

Current river rendering in drawRivers() (js/map.js:1173) draws all rivers with a single line width (tileSize * 0.18) and uses a flat color regardless of river size. This makes the Nile look identical to the Po.

Proposed Changes

1. River width classes in assets/geography.js

Add width_class to each river in RIVER_PATHS:

const RIVER_PATHS = [
  { id: 'nile',      name: 'Nile',       width_class: 'major', path: [[31,25],[31,22],...] },
  { id: 'danube',    name: 'Danube',     width_class: 'major', path: [[9,48],[14,47],...] },
  { id: 'euphrates', name: 'Euphrates',  width_class: 'major', path: [[38,38],[40,36],...] },
  { id: 'tigris',    name: 'Tigris',     width_class: 'major', path: [[43,38],[44,36],...] },
  { id: 'po',        name: 'Po',         width_class: 'minor', path: [[8,45],[10,45],[12,45]] },
  { id: 'rhone',     name: 'Rhone',      width_class: 'minor', path: [[6,46],[5,44],[5,43]] },
  { id: 'dnieper',   name: 'Dnieper',    width_class: 'minor', path: [[31,52],[31,49],[31,46]] },
  // NEW rivers:
  { id: 'volga',     name: 'Volga',      width_class: 'major', path: [[49,57],[49,53],[50,48],[51,46]] },
  { id: 'don',       name: 'Don',        width_class: 'minor', path: [[39,57],[40,54],[40,48]] },
  { id: 'jordan',    name: 'Jordan',     width_class: 'minor', path: [[35,33],[35,32],[35,31]] },
  { id: 'amu_darya', name: 'Amu Darya',  width_class: 'minor', path: [[64,38],[62,38],[60,37]] },
];

2. Width-aware drawRivers() in js/map.js

drawRivers(startX, startY, endX, endY, tileSize) {
    // major rivers: wider stroke
    // minor rivers: current thin stroke
}

3. Bridge overlay

When a road crosses a river tile, draw a bridge:

• Perpendicular stroke (90° to river flow direction) in stone color
• Only when tile.roads > 0 && isRiverTile(tile.x, tile.y)

Related

• Part of the Byzantine Ecosystem cross-repo integration plan (neobyzantine-org Phase 10)
• Pairs with #[lake-issue] (lake rendering) and #[ocean-issue] (ocean depth differentiation)