add channels to El — buffered MPMC channel with send/recv/close

Introduces Go-style channels as El's mid-flight communication primitive,
completing the threading model: threads can now not only spawn/join but
also communicate while running.

Part 1 — seed layer (el_runtime.c / el_runtime.h):
- Add __thread_create/__thread_join/__mutex_new/__mutex_lock/__mutex_unlock
  as C seed primitives (dlsym-based thread dispatch, pthread mutex table)
- Add __channel_new/__channel_send/__channel_recv/__channel_try_recv/__channel_close
  as MPMC channel seed primitives backed by mutex + condvar + circular buffer
- Bounded channels (cap > 0): circular buffer, sender blocks when full
- Unbounded channels (cap == 0): dynamic array, grows on demand, never blocks
- channel_close wakes all blocked recvers/senders; recv drains then returns ""

Part 2 — El API (runtime/channel.el):
- channel_new/send/recv/try_recv/close — thin wrappers over seed layer
- channel_pipeline — spawn N worker threads reading from in_ch, applying
  fn_name, writing to out_ch; workers exit on "" sentinel from close
- channel_drain — collect all messages from a closed channel into [String]
- channel_fan_out — send a [String] list into a channel then close it

Part 3 — codegen.el:
- Register all 10 seed builtins (__thread_* + __channel_*) in builtin_arity
  so the arity checker validates call sites at compile time

el-compiler/runtime/el_runtime.h

@@ -749,6 +749,39 @@ el_val_t trace_span_start(el_val_t name);
 el_val_t trace_span_end(el_val_t span_handle);
 el_val_t emit_event(el_val_t name, el_val_t duration_ms);
 
+/* ── Threading seed primitives ────────────────────────────────────────────────
+ * These are the low-level C primitives that back thread.el and channel.el.
+ * El programs call them via their El wrappers (spawn, join, __mutex_new, etc.)
+ * rather than directly.
+ *
+ * __thread_create(fn_name, arg) — dlsym-resolves fn_name, spawns pthread,
+ *   returns a slot index (Int) usable with __thread_join.
+ * __thread_join(tid)            — joins the thread, returns its String result.
+ * __mutex_new()                 — allocates a mutex, returns handle (Int).
+ * __mutex_lock(m)               — locks mutex m (blocks until available).
+ * __mutex_unlock(m)             — unlocks mutex m.                           */
+
+el_val_t __thread_create(el_val_t fn_name, el_val_t arg);
+el_val_t __thread_join(el_val_t tid);
+el_val_t __mutex_new(void);
+void     __mutex_lock(el_val_t m);
+void     __mutex_unlock(el_val_t m);
+
+/* ── Channel seed primitives ─────────────────────────────────────────────────
+ * Buffered MPMC channels. All values are Strings; handles are Ints.
+ *
+ * __channel_new(capacity)   — create channel; cap=0 means unbounded.
+ * __channel_send(ch, msg)   — push msg; blocks if bounded and full.
+ * __channel_recv(ch)        — pop msg; blocks until available; "" on closed+empty.
+ * __channel_try_recv(ch)    — non-blocking pop; "" if empty.
+ * __channel_close(ch)       — mark closed; wakes all blocked recvers/senders. */
+
+el_val_t __channel_new(el_val_t capacity);
+void     __channel_send(el_val_t ch, el_val_t msg);
+el_val_t __channel_recv(el_val_t ch);
+el_val_t __channel_try_recv(el_val_t ch);
+void     __channel_close(el_val_t ch);
+
 #ifdef __cplusplus
 }
 #endif