Fix: Add missing checksum field to file_meta_t and file_task_t structs

The file_meta_t and file_task_t structs were missing the checksum field
that was being used in both client and server code, causing a compilation
error. Added uint64_t checksum to both structs and updated wq_push
declaration to match implementation.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

src/utils.c

@@ -0,0 +1,130 @@
+#include "common.h"
+
+ssize_t readn(int fd, void *vptr, size_t n) {
+    size_t nleft;
+    ssize_t nread;
+    char *ptr;
+
+    ptr = vptr;
+    nleft = n;
+    while (nleft > 0) {
+        if ((nread = read(fd, ptr, nleft)) < 0) {
+            if (errno == EINTR) {
+                nread = 0;      /* call read() again */
+            } else {
+                return -1;
+            }
+        } else if (nread == 0) {
+            break;              /* EOF */
+        }
+        nleft -= nread;
+        ptr   += nread;
+    }
+    return n - nleft;
+}
+
+ssize_t writen(int fd, const void *vptr, size_t n) {
+    size_t nleft;
+    ssize_t nwritten;
+    const char *ptr;
+
+    ptr = vptr;
+    nleft = n;
+    while (nleft > 0) {
+        if ((nwritten = write(fd, ptr, nleft)) <= 0) {
+            if (nwritten < 0 && errno == EINTR)
+                nwritten = 0;   /* and call write() again */
+            else
+                return -1;     /* error */
+        }
+        nleft  -= nwritten;
+        ptr    += nwritten;
+    }
+    return n;
+}
+
+// ─── Simple path creation ───────────────────────────────────────────────
+int make_path(const char *path, mode_t mode) {
+    char tmp[2048], *p = NULL;
+    size_t len;
+
+    snprintf(tmp, sizeof(tmp), "%s", path);
+    len = strlen(tmp);
+    if (tmp[len - 1] == '/')
+        tmp[len - 1] = '\0';
+    for (p = tmp + 1; *p; p++)
+        if (*p == '/') {
+            *p = '\0';
+            if (mkdir(tmp, mode) == -1 && errno != EEXIST)
+                return -1;
+            *p = '/';
+        }
+    if (mkdir(tmp, mode) == -1 && errno != EEXIST)
+        return -1;
+    return 0;
+}
+
+// ─── Work-Queue (client-side, thread-safe) ───────────────────────────────────
+
+void wq_init(work_queue_t *q) {
+    q->head  = NULL;
+    q->tail  = NULL;
+    q->done  = 0;
+    q->error = 0;
+    pthread_mutex_init(&q->lock, NULL);
+    pthread_cond_init(&q->cond, NULL);
+}
+
+void wq_push(work_queue_t *q, const char *full_path, const char *rel_path, uint64_t file_size, uint64_t checksum) {
+    file_task_t *t = malloc(sizeof(file_task_t));
+    if (!t) { perror("wq_push malloc"); return; }
+    snprintf(t->full_path, sizeof(t->full_path), "%s", full_path);
+    snprintf(t->rel_path,  sizeof(t->rel_path),  "%s", rel_path);
+    t->file_size = file_size;
+    t->checksum = checksum;
+    t->next = NULL;
+
+    pthread_mutex_lock(&q->lock);
+    if (q->tail) {
+        q->tail->next = t;
+    } else {
+        q->head = t;
+    }
+    q->tail = t;
+    pthread_cond_signal(&q->cond);
+    pthread_mutex_unlock(&q->lock);
+}
+
+/* Returns NULL when queue is empty AND done=1 (no more work ever). */
+file_task_t *wq_pop(work_queue_t *q) {
+    pthread_mutex_lock(&q->lock);
+    while (q->head == NULL && !q->done) {
+        pthread_cond_wait(&q->cond, &q->lock);
+    }
+    file_task_t *t = q->head;
+    if (t) {
+        q->head = t->next;
+        if (!q->head) q->tail = NULL;
+    }
+    pthread_mutex_unlock(&q->lock);
+    return t;   /* NULL means done */
+}
+
+void wq_finish(work_queue_t *q) {
+    pthread_mutex_lock(&q->lock);
+    q->done = 1;
+    pthread_cond_broadcast(&q->cond);   /* wake ALL waiting workers */
+    pthread_mutex_unlock(&q->lock);
+}
+
+void wq_destroy(work_queue_t *q) {
+    /* drain any leftover tasks */
+    file_task_t *t = q->head;
+    while (t) {
+        file_task_t *next = t->next;
+        free(t);
+        t = next;
+    }
+    pthread_mutex_destroy(&q->lock);
+    pthread_cond_destroy(&q->cond);
+}