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.c

@@ -102,6 +102,45 @@ void el_request_end(void) {
     _tl_arena.count = 0;
 }
 
+/* ── Scoped arena for CLI use ─────────────────────────────────────────────── *
+ * CLI programs never call el_request_start/end, so all strdup allocations are
+ * permanent. el_arena_push/pop let the compiler free intermediate strings
+ * after each compilation unit.
+ *
+ * el_arena_push() — activates the arena if not already active, saves the
+ *   current arena count as a mark, and returns it as an el_val_t Int.
+ * el_arena_pop(mark) — frees all strings allocated since the push mark and
+ *   resets the count. If count reaches 0, deactivates the arena.
+ */
+#define EL_ARENA_SCOPE_DEPTH 32
+static _Thread_local size_t  _tl_arena_scope[EL_ARENA_SCOPE_DEPTH];
+static _Thread_local int     _tl_arena_scope_depth = 0;
+
+el_val_t el_arena_push(void) {
+    if (!_tl_arena_active) {
+        _tl_arena_active = 1;
+    }
+    if (_tl_arena_scope_depth < EL_ARENA_SCOPE_DEPTH) {
+        _tl_arena_scope[_tl_arena_scope_depth++] = _tl_arena.count;
+    }
+    return (el_val_t)(int64_t)_tl_arena.count;
+}
+
+el_val_t el_arena_pop(el_val_t mark) {
+    size_t save = (size_t)(int64_t)mark;
+    if (save > _tl_arena.count) save = 0;
+    for (size_t i = save; i < _tl_arena.count; i++) {
+        if (_tl_arena.ptrs[i]) {
+            free(_tl_arena.ptrs[i]);
+            _tl_arena.ptrs[i] = NULL;
+        }
+    }
+    _tl_arena.count = save;
+    if (_tl_arena_scope_depth > 0) _tl_arena_scope_depth--;
+    if (save == 0) _tl_arena_active = 0;
+    return 0;
+}
+
 /* Persistent allocation — bypasses the arena (state_set, engram internals). */
 static char* el_strdup_persist(const char* s) {
     if (!s) return strdup("");