refactor(core)!: Add context, config, and structured logging

Introduces `context.Context` to the `FetchCourse` method and its call chain, allowing for cancellable network requests and timeouts. This improves application robustness when fetching remote course data.

A new configuration package centralizes application settings, loading them from environment variables with sensible defaults for base URL, request timeout, and logging.

Standard `log` and `fmt` calls are replaced with a structured logging system built on `slog`, supporting both JSON and human-readable text formats.

This change also includes:
- Extensive benchmarks and example tests.
- Simplified Go doc comments across several packages.

BREAKING CHANGE: The `NewArticulateParser` constructor signature has been updated to accept a logger, base URL, and timeout, which are now supplied via the new configuration system.

internal/config/config.go

@@ -0,0 +1,77 @@
+// Package config provides configuration management for the articulate-parser application.
+// It supports loading configuration from environment variables and command-line flags.
+package config
+
+import (
+	"log/slog"
+	"os"
+	"strconv"
+	"time"
+)
+
+// Config holds all configuration values for the application.
+type Config struct {
+	// Parser configuration
+	BaseURL        string
+	RequestTimeout time.Duration
+
+	// Logging configuration
+	LogLevel  slog.Level
+	LogFormat string // "json" or "text"
+}
+
+// Default configuration values.
+const (
+	DefaultBaseURL        = "https://rise.articulate.com"
+	DefaultRequestTimeout = 30 * time.Second
+	DefaultLogLevel       = slog.LevelInfo
+	DefaultLogFormat      = "text"
+)
+
+// Load creates a new Config with values from environment variables.
+// Falls back to defaults if environment variables are not set.
+func Load() *Config {
+	return &Config{
+		BaseURL:        getEnv("ARTICULATE_BASE_URL", DefaultBaseURL),
+		RequestTimeout: getDurationEnv("ARTICULATE_REQUEST_TIMEOUT", DefaultRequestTimeout),
+		LogLevel:       getLogLevelEnv("LOG_LEVEL", DefaultLogLevel),
+		LogFormat:      getEnv("LOG_FORMAT", DefaultLogFormat),
+	}
+}
+
+// getEnv retrieves an environment variable or returns the default value.
+func getEnv(key, defaultValue string) string {
+	if value := os.Getenv(key); value != "" {
+		return value
+	}
+	return defaultValue
+}
+
+// getDurationEnv retrieves a duration from environment variable or returns default.
+// The environment variable should be in seconds (e.g., "30" for 30 seconds).
+func getDurationEnv(key string, defaultValue time.Duration) time.Duration {
+	if value := os.Getenv(key); value != "" {
+		if seconds, err := strconv.Atoi(value); err == nil {
+			return time.Duration(seconds) * time.Second
+		}
+	}
+	return defaultValue
+}
+
+// getLogLevelEnv retrieves a log level from environment variable or returns default.
+// Accepts: "debug", "info", "warn", "error" (case-insensitive).
+func getLogLevelEnv(key string, defaultValue slog.Level) slog.Level {
+	value := os.Getenv(key)
+	switch value {
+	case "debug", "DEBUG":
+		return slog.LevelDebug
+	case "info", "INFO":
+		return slog.LevelInfo
+	case "warn", "WARN", "warning", "WARNING":
+		return slog.LevelWarn
+	case "error", "ERROR":
+		return slog.LevelError
+	default:
+		return defaultValue
+	}
+}