runtime: add EL_TRUE/EL_FALSE macros and scoped arena for CLI

Adds EL_TRUE/EL_FALSE convenience macros to el_runtime.h alongside the
existing EL_NULL, making boolean-returning builtins readable without
raw (el_val_t) casts. Documents all value macros in the header comment.

Also lands el_arena_push/el_arena_pop — a scoped string arena for CLI
programs that never call el_request_start/end. The compiler can push a
mark before a compilation unit and pop it after to free intermediate
strings, reducing peak RSS during long compile runs.

lang/el-compiler/runtime/el_runtime.h

@@ -22,6 +22,9 @@
  *   EL_STR(s)    cast string literal to el_val_t
  *   EL_CSTR(v)   cast el_val_t back to const char*
  *   EL_INT(v)    identity — el_val_t is already int64_t
+ *   EL_NULL      null / zero value
+ *   EL_FALSE     boolean false (0)
+ *   EL_TRUE      boolean true (1)
  *
  * Link requirements:
  *   -lcurl     — required for the HTTP client (http_get, http_post, llm_*).
@@ -53,6 +56,8 @@ typedef int64_t el_val_t;
 #define EL_CSTR(v) ((const char*)(uintptr_t)(v))
 #define EL_INT(v)  (v)
 #define EL_NULL    ((el_val_t)0)
+#define EL_FALSE   ((el_val_t)0)
+#define EL_TRUE    ((el_val_t)1)
 
 /* Float values share the el_val_t (int64) slot via a bit-cast.
  * The codegen emits Float literals as `el_from_float(<dbl>)` so the
@@ -117,6 +122,10 @@ el_val_t  el_min(el_val_t a, el_val_t b);
 void      el_retain(el_val_t v);
 void      el_release(el_val_t v);
 
+/* ── Scoped arena (CLI use) ───────────────────────────────────────────────── */
+el_val_t  el_arena_push(void);
+el_val_t  el_arena_pop(el_val_t mark);
+
 /* ── List ────────────────────────────────────────────────────────────────── */
 
 el_val_t  el_list_new(el_val_t count, ...);