Fix coastline: 2-edge only, explicit waterSide, flip control

Coastline analysis and fix:
- Coastlines now only show 2-edge patterns (straight, wide curve,
  sharp bend) — Y-junctions and crossroads removed since a coast
  always divides exactly two sides
- Added waterSide property to HexFeature (1=CW, -1=CCW) — stored
  explicitly instead of computed by broken center-distance heuristic
  that flipped water/land at rotation midpoint
- F key or "Flip water side" button in palette to toggle which side
  is water before placing
- Inspector shows flip button (swap arrows) for placed coastlines
- Rotation preserves waterSide — no more land/water swaps
- Separate pattern lists: GENERAL_PATTERNS for road/river,
  COASTLINE_PATTERNS for coastline (core/tile-patterns.ts)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/hex-map.ts

@@ -42,13 +42,14 @@ export class HexMap {
   }
 
   /** Add or update a linear feature on a hex */
-  setFeature(coord: AxialCoord, terrainId: string, edgeMask: EdgeMask): void {
+  setFeature(coord: AxialCoord, terrainId: string, edgeMask: EdgeMask, waterSide?: 1 | -1): void {
     const terrain = this.getTerrain(coord);
     const existing = terrain.features.find(f => f.terrainId === terrainId);
     if (existing) {
       existing.edgeMask = edgeMask;
+      if (waterSide !== undefined) existing.waterSide = waterSide;
     } else {
-      terrain.features.push({ terrainId, edgeMask });
+      terrain.features.push({ terrainId, edgeMask, ...(waterSide !== undefined ? { waterSide } : {}) });
     }
     // Remove features with empty mask
     terrain.features = terrain.features.filter(f => f.edgeMask !== 0);