el runtime: prototype + fix channel/mutex seed ABI for modern clang

el_runtime.h declared only __thread_create/__thread_join; the mutex and
channel seed primitives (__mutex_*, __channel_*) were defined in
el_runtime.c but never prototyped. Under Apple clang 21 (C11) the missing
prototypes became implicit-declaration errors, and the void-returning
__channel_send/__channel_close mis-typed el_val_t (long long) returns,
so any El program using runtime/channel.el failed to compile.

- add prototypes for __mutex_new/lock/unlock and all __channel_* to el_runtime.h
- make __channel_send/__channel_close return el_val_t nil so elc's
  trailing-expression codegen for the void El wrappers type-checks

Additive; unbreaks native channels for every downstream El program.

lang/el-compiler/runtime/el_runtime.c

@@ -11743,9 +11743,9 @@ el_val_t __channel_new(el_val_t capacity_v) {
     return EL_INT(slot);
 }
 
-void __channel_send(el_val_t ch_v, el_val_t msg_v) {
+el_val_t __channel_send(el_val_t ch_v, el_val_t msg_v) {
     int slot = (int)(int64_t)ch_v;
-    if (slot < 0 || slot >= EL_CHANNEL_MAX) return;
+    if (slot < 0 || slot >= EL_CHANNEL_MAX) return EL_STR("");
     ElChannel* ch = &_channels[slot];
 
     const char* msg = EL_CSTR(msg_v);
@@ -11758,7 +11758,7 @@ void __channel_send(el_val_t ch_v, el_val_t msg_v) {
         /* Send on closed channel is a no-op (drop the message). */
         pthread_mutex_unlock(&ch->mu);
         free(copy);
-        return;
+        return EL_STR("");
     }
 
     if (ch->cap > 0) {
@@ -11769,7 +11769,7 @@ void __channel_send(el_val_t ch_v, el_val_t msg_v) {
         if (ch->closed) {
             pthread_mutex_unlock(&ch->mu);
             free(copy);
-            return;
+            return EL_STR("");
         }
         ch->buf[ch->tail] = copy;
         ch->tail = (ch->tail + 1) % ch->cap;
@@ -11783,7 +11783,7 @@ void __channel_send(el_val_t ch_v, el_val_t msg_v) {
                 pthread_mutex_unlock(&ch->mu);
                 free(copy);
                 fprintf(stderr, "[__channel_send] out of memory growing channel\n");
-                return;
+                return EL_STR("");
             }
             /* The circular buffer may have wrapped. Linearise it first.
              * In unbounded mode head is always 0 (we append at tail, drain
@@ -11807,6 +11807,7 @@ void __channel_send(el_val_t ch_v, el_val_t msg_v) {
 
     pthread_cond_signal(&ch->not_empty);
     pthread_mutex_unlock(&ch->mu);
+    return EL_STR("");
 }
 
 el_val_t __channel_recv(el_val_t ch_v) {
@@ -11864,9 +11865,9 @@ el_val_t __channel_try_recv(el_val_t ch_v) {
     return EL_STR(msg);
 }
 
-void __channel_close(el_val_t ch_v) {
+el_val_t __channel_close(el_val_t ch_v) {
     int slot = (int)(int64_t)ch_v;
-    if (slot < 0 || slot >= EL_CHANNEL_MAX) return;
+    if (slot < 0 || slot >= EL_CHANNEL_MAX) return EL_STR("");
     ElChannel* ch = &_channels[slot];
 
     pthread_mutex_lock(&ch->mu);
@@ -11875,6 +11876,7 @@ void __channel_close(el_val_t ch_v) {
     pthread_cond_broadcast(&ch->not_empty);
     pthread_cond_broadcast(&ch->not_full);
     pthread_mutex_unlock(&ch->mu);
+    return EL_STR("");
 }
 
 /* ── DHARMA runtime additions ────────────────────────────────────────────────

lang/el-compiler/runtime/el_runtime.h

@@ -803,6 +803,19 @@ el_val_t emit_event(el_val_t name, el_val_t duration_ms);
 el_val_t __thread_create(el_val_t fn_name_v, el_val_t arg_v);
 el_val_t __thread_join(el_val_t tid_v);
 
+/* Mutex + channel seed primitives (defined in el_runtime.c). Declared here so
+ * that compiled El programs which use runtime/thread.el's with_mutex helper or
+ * runtime/channel.el's Go-style channels see real prototypes instead of an
+ * implicit int-return declaration (which the C11 ABI mis-truncates el_val_t). */
+el_val_t __mutex_new(void);
+void     __mutex_lock(el_val_t m_v);
+void     __mutex_unlock(el_val_t m_v);
+el_val_t __channel_new(el_val_t capacity_v);
+el_val_t __channel_send(el_val_t ch_v, el_val_t msg_v);
+el_val_t __channel_recv(el_val_t ch_v);
+el_val_t __channel_try_recv(el_val_t ch_v);
+el_val_t __channel_close(el_val_t ch_v);
+
 /* ── __ prefixed aliases (self-hosting compiler ABI) ─────────────────────────
  * The El self-hosting compiler emits calls to __-prefixed names. These are
  * forwarding wrappers around the existing el_runtime functions above.        */