core: add timestamped klog logger on VGA console

include/klog.h

@@ -0,0 +1,48 @@
+/* Nadir boot logger: timestamped `[    0.000000] sub: msg` lines.
+ *
+ * Runs in 64-bit long mode at Ring 0 (CPL 0) on the VGA console via
+ * console_putchar, which is polling-only and IRQ-safe by construction.
+ *
+ * Timestamp source: pit_ticks() / PIT_HZ (100Hz, 10ms per tick), integer
+ * math only. At 100Hz one
+ * tick is 10ms, so the 6-digit fraction only changes in steps of 10000
+ * (e.g. 0.010000); the last 4 digits are always zero by construction.
+ * Early boot note: IF=0 until kmain runs `sti`, so the PIT IRQ cannot fire
+ * and ticks stay frozen at 0.
+ *
+ * IF/IRQ safety: mainline only, never IRQ context. The PIT IRQ handler
+ * (pit_on_tick) must never call this. It only bumps the tick counter.
+ * Single-core kernel, so no locking is needed.
+ *
+ * Preconditions: console is usable (identity-mapped VGA); pit_ticks() is
+ * linked in (BSS-zeroed counter reads 0 before pit_init).
+ * Failure modes: none
+ */
+
+#ifndef NADIR_KLOG_H
+#define NADIR_KLOG_H
+
+#include <stdint.h>
+
+/* Print one full line: `[    0.000000] subsystem: msg` + '\n'. */
+void klog(const char *subsystem, const char *msg);
+
+/* Piecewise line builder for values (PMM stats, uptime): prints the
+ * `[    0.000000] subsystem: ` prefix with no newline; append with
+ * klog_str/dec/hex64, then close with klog_end() ('\n').
+ * Preconditions: calls nest as begin -> (str|dec|hex64)* -> end. */
+void klog_begin(const char *subsystem);
+
+/* Append a NUL-terminated string to the open line. */
+void klog_str(const char *s);
+
+/* Append an unsigned decimal value (no libc printf). */
+void klog_dec(uint64_t v);
+
+/* Append a 64-bit value as 0x-prefixed, zero-padded hexadecimal. */
+void klog_hex64(uint64_t v);
+
+/* Terminate the open line with '\n'. */
+void klog_end(void);
+
+#endif /* NADIR_KLOG_H */