refactor: comprehensive code quality improvements

- Add logger to all exporters for consistent error handling
- Add Course.Validate() method with ValidationError type
- Create shared test fixtures in internal/testdata
- Refactor config passing: NewArticulateParser now takes *config.Config
- Add NewLoggerFromConfig() factory for logger creation
- Add context cancellation with signal handling (SIGINT/SIGTERM)
- Add godoc examples for loggers and exporters
- Add HTML exporter error path tests
- Add fuzz tests for parser and HTMLCleaner
- Fix markdown output: trim whitespace, collapse blank lines
- Remove test output files from repo

internal/services/logger_factory.go

@@ -0,0 +1,15 @@
+package services
+
+import (
+	"github.com/kjanat/articulate-parser/internal/config"
+	"github.com/kjanat/articulate-parser/internal/interfaces"
+)
+
+// NewLoggerFromConfig creates a logger based on the configuration.
+// Returns a JSON logger if cfg.LogFormat is "json", otherwise returns a text logger.
+func NewLoggerFromConfig(cfg *config.Config) interfaces.Logger {
+	if cfg.LogFormat == "json" {
+		return NewSlogLogger(cfg.LogLevel)
+	}
+	return NewTextLogger(cfg.LogLevel)
+}