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.

src/stk_log.c

@@ -1,14 +1,86 @@
 #include "stk_log.h"
+#include "stk.h"
 #include <stdarg.h>
 #include <stdio.h>
+#include <string.h>
 
-void stk_log(FILE *fp, const char *fmt, ...)
+#define STK_LOG_TIMESTAMP_BUFFER 32
+
+extern unsigned char stk_flags;
+void platform_get_timestamp(char *buffer, size_t size);
+
+static FILE *log_output = NULL;
+static char log_prefix[STK_LOG_PREFIX_BUFFER] = "stk";
+static stk_log_level_t min_log_level = STK_LOG_INFO;
+
+static const char *get_level_string(stk_log_level_t level)
 {
-	va_list args;
-	va_start(args, fmt);
+	char *level_str = "";
+	switch (level) {
+	case STK_LOG_ERROR:
+		level_str = "ERROR";
+		break;
+	case STK_LOG_WARN:
+		level_str = "WARN";
+		break;
+	case STK_LOG_INFO:
+		level_str = "INFO";
+		break;
+	case STK_LOG_DEBUG:
+		level_str = "DEBUG";
+		break;
+	}
 
-	vfprintf(fp, fmt, args);
-	fputc('\n', fp);
+	return level_str;
+}
 
+void stk_set_log_output(FILE *fp)
+{
+	if (fp == NULL)
+		stk_flags &= ~STK_FLAG_LOGGING_ENABLED;
+
+	log_output = fp;
+}
+
+void stk_set_log_prefix(const char *prefix)
+{
+	if (!prefix) {
+		log_prefix[0] = '\0';
+		return;
+	}
+
+	strncpy(log_prefix, prefix, STK_LOG_PREFIX_BUFFER - 1);
+	log_prefix[STK_LOG_PREFIX_BUFFER - 1] = '\0';
+}
+
+void stk_set_log_level(stk_log_level_t level) { min_log_level = level; }
+
+void stk_log(stk_log_level_t level, const char *fmt, ...)
+{
+	FILE *output;
+	const char *level_str;
+	char timestamp[STK_LOG_TIMESTAMP_BUFFER];
+	va_list args;
+
+	if (!(stk_flags & STK_FLAG_LOGGING_ENABLED))
+		return;
+
+	if (level < min_log_level)
+		return;
+
+	output = log_output ? log_output : stdout;
+	level_str = get_level_string(level);
+	platform_get_timestamp(timestamp, sizeof(timestamp));
+
+	if (log_prefix[0] != '\0')
+		fprintf(output, "%s [%s] [%s] ", timestamp, log_prefix,
+			level_str);
+	else
+		fprintf(output, "%s [%s] ", timestamp, level_str);
+
+	va_start(args, fmt);
+	vfprintf(output, fmt, args);
 	va_end(args);
+
+	fputc('\n', output);
 }