feat: Implement METAR runway visual range (RVR) decoding

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

metar.html

@@ -310,6 +310,42 @@
             return `${code}: Altimeter ${alt.slice(0, 2)}.${alt.slice(2)} inches of mercury`;
         }
         
+        function decodeRunwayInfo(code) {
+            const parts = code.split('/');
+            const runway = `Runway ${parts[0].substring(1)}`;
+
+            const visPart = parts[1];
+            let visText;
+
+            function decodeVis(visStr) {
+                if (visStr.startsWith('P')) return `greater than ${visStr.substring(1)}`;
+                if (visStr.startsWith('M')) return `less than ${visStr.substring(1)}`;
+                return visStr;
+            }
+
+            if (visPart.includes('V')) {
+                const [minVis, maxVis] = visPart.replace('FT', '').split('V');
+                visText = `visual range variable between ${decodeVis(minVis)} and ${decodeVis(maxVis)} feet`;
+            } else {
+                const vis = visPart.replace('FT', '');
+                visText = `visual range ${decodeVis(vis)} feet`;
+            }
+
+            let trendText = '';
+            if (parts.length > 2 && parts[2]) {
+                const trendCode = parts[2];
+                const trendMap = {
+                    'N': 'not significant',
+                    'U': 'upward',
+                    'D': 'downward'
+                };
+                const trendDesc = trendMap[trendCode] || 'unknown';
+                trendText = `, trend ${trendDesc} (${trendCode})`;
+            }
+
+            return `${code}: ${runway} ${visText}${trendText}`;
+        }
+        
         function decodeSlp(code) {
             const pressure = parseInt(code.substring(3));
             const hpa = (pressure < 500 ? 1000 : 900) + (pressure / 10);
@@ -701,6 +737,8 @@
                     sections.push({ raw: part, decoded: decodeTempDew(part) });
                 } else if (part.startsWith('A') && part.length === 5 && !isNaN(part.substring(1))) {
                     sections.push({ raw: part, decoded: decodeAltimeter(part) });
+                } else if (part.match(/^R\d{2}[LRC]?\//)) {
+                    sections.push({ raw: part, decoded: decodeRunwayInfo(part) });
                 } else {
                     console.log(`Unknown main METAR part: ${part} in METAR: ${metarString}`);
                     sections.push({ raw: part, decoded: `${part}: Unknown METAR component` });