perf(matmul): bypass printf — direct UART 16550 register writes

Replace all printf() calls with direct UART THR register access:
- uart_tx(): poll LSR bit5, write byte to THR
- uart_puts(): null-terminated string output
- uart_puthex(): 8-digit lowercase hex for CRC32

Eliminates picolibc printf overhead: no format parsing, no FILE* I/O,
no locale, no buffering. UART runs at 115200 baud (~8.7μs/byte),
THR nearly always empty at 33MHz core → single poll iteration.

Output: MATMUL_START, MATMUL_CRC32=XXXXXXXX, MATMUL_DONE

sdk/software/examples/asm/user-sample.c

@@ -10,11 +10,32 @@
  *   4. XOR bank toggle      → avoids g%2 recomputation per iteration
  */
 
-#include <stdio.h>
 #include <stdint.h>
 
 #include <dma.h>
 
+/* ─── UART 16550 direct access (bypasses slow printf/picolibc) ──────── */
+#define UART_THR (*(volatile uint8_t *)0xbf000000)
+#define UART_LSR (*(volatile uint8_t *)0xbf000005)
+#define LSR_THRE  0x20
+
+static inline void uart_tx(uint8_t c) {
+    while (!(UART_LSR & LSR_THRE)) {}
+    UART_THR = c;
+}
+
+static void uart_puts(const char *s) {
+    while (*s) uart_tx(*s++);
+}
+
+static void uart_puthex(uint32_t val) {
+    int i;
+    for (i = 28; i >= 0; i -= 4) {
+        uint8_t nibble = (val >> i) & 0xF;
+        uart_tx(nibble < 10 ? '0' + nibble : 'a' + nibble - 10);
+    }
+}
+
 /* ─── Configurable group count (default 5000) ──────────────────────── */
 #ifndef MATMUL_GROUP_NUM
 #define MATMUL_GROUP_NUM 5000
@@ -80,7 +101,7 @@ int main(void)
     uint32_t ctrl_start[2]   = {CTRL_START_B0, CTRL_START_B1};
 
     /* ── Start marker (must be first output) ──────────────────── */
-    printf("MATMUL_START\n");
+    uart_puts("MATMUL_START\r\n");
 
     // Reset IP state
     *mat_status = STATUS_DONE | STATUS_ERROR;
@@ -186,8 +207,9 @@ int main(void)
     /* ── Read hardware CRC32 ──────────────────────────────────── */
     crc = *mat_crc;
 
-    printf("MATMUL_CRC32=%08lx\n", (unsigned long)crc);
+    uart_puts("MATMUL_CRC32=");
-    printf("MATMUL_DONE\n");
+    uart_puthex(crc);
+    uart_puts("\r\nMATMUL_DONE\r\n");
 
     return 0;
 }