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/src/codegen.el

@@ -2111,6 +2111,18 @@ fn builtin_arity(name: String) -> Int {
     if str_eq(name, "get") { return 2 }
     if str_eq(name, "map_get") { return 2 }
     if str_eq(name, "map_set") { return 3 }
+    // Threading seed primitives
+    if str_eq(name, "__thread_create") { return 2 }
+    if str_eq(name, "__thread_join") { return 1 }
+    if str_eq(name, "__mutex_new") { return 0 }
+    if str_eq(name, "__mutex_lock") { return 1 }
+    if str_eq(name, "__mutex_unlock") { return 1 }
+    // Channel seed primitives
+    if str_eq(name, "__channel_new") { return 1 }
+    if str_eq(name, "__channel_send") { return 2 }
+    if str_eq(name, "__channel_recv") { return 1 }
+    if str_eq(name, "__channel_try_recv") { return 1 }
+    if str_eq(name, "__channel_close") { return 1 }
     // -1 sentinel: variadic / unknown / user-defined -> no check.
     return -1
 }