feat: add command queue and FIFO pipe for unified input handling

Co-authored-by: aider (deepseek/deepseek-reasoner) <aider@aider.chat>

src/queue.h

@@ -0,0 +1,31 @@
+#ifndef QUEUE_H
+#define QUEUE_H
+
+#include "command.h"
+#include <stdbool.h>
+
+/* Fixed‑size lock‑free SPSC queue (single producer, single consumer).
+ * The queue is safe for one thread writing (producer) and one thread
+ * reading (consumer).  No locks, no dynamic memory allocation.
+ * Must be initialised before first use.  All operations are RT‑safe. */
+
+#define QUEUE_CAPACITY 256
+
+typedef struct {
+    command_t  buffer[QUEUE_CAPACITY];
+    /* head: index where next element will be written (producer only)
+     * tail: index of next element to read (consumer only) */
+    int head;
+    int tail;
+} spsc_queue_t;
+
+/* Initialise queue (must be called once before any push/pop). */
+void queue_init(spsc_queue_t *q);
+
+/* Push a command.  Returns true on success, false if queue full. */
+bool queue_push(spsc_queue_t *q, command_t cmd);
+
+/* Pop a command.  Returns true if a command was retrieved, false if empty. */
+bool queue_pop(spsc_queue_t *q, command_t *cmd);
+
+#endif