feature: add CoinGecko cryptocurrency price extractor

Add sites/coingecko package with GetPrice() method that extracts
structured crypto price data (name, symbol, price, 24h/7d change,
market cap, volume, 24h high/low) from CoinGecko coin pages.

Includes mock-based tests and parseLargeNumber helper for T/B/M suffixes.

Closes #27

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

sites/coingecko/coingecko.go

@@ -0,0 +1,169 @@
+package coingecko
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"strings"
+	"time"
+
+	"gitea.stevedudenhoeffer.com/steve/go-extractor"
+	"gitea.stevedudenhoeffer.com/steve/go-extractor/sites/internal/parse"
+)
+
+// CoinPrice holds structured cryptocurrency price data from CoinGecko.
+type CoinPrice struct {
+	Name      string
+	Symbol    string
+	Price     float64
+	Change24h float64 // percentage
+	Change7d  float64
+	MarketCap float64
+	Volume24h float64
+	High24h   float64
+	Low24h    float64
+}
+
+// Config holds configuration for the CoinGecko extractor.
+type Config struct{}
+
+// DefaultConfig is the default CoinGecko configuration.
+var DefaultConfig = Config{}
+
+func (c Config) validate() Config {
+	return c
+}
+
+// coinURL returns the CoinGecko URL for a given coin ID (e.g. "bitcoin", "ethereum").
+func coinURL(coin string) string {
+	return fmt.Sprintf("https://www.coingecko.com/en/coins/%s", strings.ToLower(coin))
+}
+
+// GetPrice extracts structured cryptocurrency price data from CoinGecko.
+func (c Config) GetPrice(ctx context.Context, b extractor.Browser, coin string) (*CoinPrice, error) {
+	c = c.validate()
+
+	u := coinURL(coin)
+
+	slog.Info("fetching coin price", "url", u, "coin", coin)
+	doc, err := b.Open(ctx, u, extractor.OpenPageOptions{})
+	if err != nil {
+		return nil, fmt.Errorf("failed to open coin page: %w", err)
+	}
+	defer extractor.DeferClose(doc)
+
+	timeout := 10 * time.Second
+	if err := doc.WaitForNetworkIdle(&timeout); err != nil {
+		slog.Warn("WaitForNetworkIdle failed", "err", err)
+	}
+
+	return extractCoinPrice(doc)
+}
+
+// GetPrice is a convenience function using DefaultConfig.
+func GetPrice(ctx context.Context, b extractor.Browser, coin string) (*CoinPrice, error) {
+	return DefaultConfig.GetPrice(ctx, b, coin)
+}
+
+func extractCoinPrice(doc extractor.Node) (*CoinPrice, error) {
+	var data CoinPrice
+
+	// Coin name — typically in h1 on the coin page
+	names := doc.Select("div[data-coin-show-target='coin'] h1")
+	if len(names) == 0 {
+		names = doc.Select("h1")
+	}
+	if len(names) > 0 {
+		data.Name, _ = names[0].Text()
+		data.Name = strings.TrimSpace(data.Name)
+	}
+
+	// Symbol — often shown as abbreviated ticker
+	syms := doc.Select("span[data-coin-symbol]")
+	if len(syms) > 0 {
+		txt, _ := syms[0].Text()
+		data.Symbol = strings.ToUpper(strings.TrimSpace(txt))
+	}
+
+	// Current price
+	prices := doc.Select("span[data-converter-target='price']")
+	if len(prices) == 0 {
+		prices = doc.Select("span[data-price-target='price']")
+	}
+	if len(prices) > 0 {
+		txt, _ := prices[0].Text()
+		data.Price = parse.NumericOnly(txt)
+	}
+
+	// 24h change percentage
+	changes24h := doc.Select("span[data-price-change='24h']")
+	if len(changes24h) > 0 {
+		txt, _ := changes24h[0].Text()
+		val := parse.NumericOnly(txt)
+		if strings.Contains(txt, "-") {
+			val = -val
+		}
+		data.Change24h = val
+	}
+
+	// 7d change percentage
+	changes7d := doc.Select("span[data-price-change='7d']")
+	if len(changes7d) > 0 {
+		txt, _ := changes7d[0].Text()
+		val := parse.NumericOnly(txt)
+		if strings.Contains(txt, "-") {
+			val = -val
+		}
+		data.Change7d = val
+	}
+
+	// Market cap
+	mcaps := doc.Select("span[data-coin-stat='market-cap']")
+	if len(mcaps) > 0 {
+		txt, _ := mcaps[0].Text()
+		data.MarketCap = parseLargeNumber(txt)
+	}
+
+	// 24h volume
+	vols := doc.Select("span[data-coin-stat='volume']")
+	if len(vols) > 0 {
+		txt, _ := vols[0].Text()
+		data.Volume24h = parseLargeNumber(txt)
+	}
+
+	// 24h high
+	highs := doc.Select("span[data-coin-stat='high-24h']")
+	if len(highs) > 0 {
+		txt, _ := highs[0].Text()
+		data.High24h = parse.NumericOnly(txt)
+	}
+
+	// 24h low
+	lows := doc.Select("span[data-coin-stat='low-24h']")
+	if len(lows) > 0 {
+		txt, _ := lows[0].Text()
+		data.Low24h = parse.NumericOnly(txt)
+	}
+
+	return &data, nil
+}
+
+// parseLargeNumber handles values like "$1,234,567,890" or "$1.5T" or "$250B".
+func parseLargeNumber(s string) float64 {
+	s = strings.TrimSpace(s)
+	upper := strings.ToUpper(s)
+
+	numeric := parse.NumericOnly(s)
+
+	if strings.HasSuffix(upper, "T") {
+		return numeric * 1_000_000_000_000
+	}
+	if strings.HasSuffix(upper, "B") {
+		return numeric * 1_000_000_000
+	}
+	if strings.HasSuffix(upper, "M") {
+		return numeric * 1_000_000
+	}
+
+	return numeric
+}