feat(logging)!: add enhanced logging system

BREAKING CHANGE: stk_log() signature changed to require log level

- Add log levels (ERROR, WARN, INFO, DEBUG) with runtime filtering
- Add timestamps (yyyy-mm-dd HH:MM:SS.mmm format)
- Add stk_set_log_output(), stk_set_log_prefix(), stk_set_log_level()
- Platform-specific timestamp: GetLocalTime (Windows), gettimeofday (POSIX)
- Consolidate duplicate platform includes into POSIX common block
- Setting log output to NULL disables logging.

Default: INFO level, "stk" prefix, stdout output. All internal messages
use appropriate severity levels. Single byte flag controls enable/disable.

include/stk_log.h

@@ -7,7 +7,18 @@
 extern "C" {
 #endif
 
-void stk_log(FILE *fp, const char *fmt, ...);
+typedef enum {
+	STK_LOG_ERROR,
+	STK_LOG_WARN,
+	STK_LOG_INFO,
+	STK_LOG_DEBUG
+} stk_log_level_t;
+
+void stk_set_log_output(FILE *fp);
+void stk_set_log_prefix(const char *prefix);
+void stk_set_log_level(stk_log_level_t min_level);
+
+void stk_log(stk_log_level_t level, const char *fmt, ...);
 
 #ifdef __cplusplus
 }