runtime: restore el_runtime.c as build shim; fix el_seed.h self-contained types

el_runtime.c was deleted prematurely — elb still resolves the runtime at build time
via a hardcoded relative path, and the elc code generator still emits
#include "el_runtime.h" in generated C.

Restoring el_runtime.c + el_runtime.h as the working build runtime until the
compiler is updated to emit #include "el_seed.h" and link against el_seed.c
directly.

el_seed.h: remove #include "el_runtime.h" that broke after el_runtime.h deletion;
add inline el_val_t typedef + macros + float cast helpers so el_seed.h is fully
self-contained.

el-compiler/runtime/el_seed.h

@@ -1,27 +1,48 @@
 /*
  * el_seed.h — El language seed runtime header
  *
- * Declares all OS-boundary primitives available to new-generation El programs.
+ * Declares all OS-boundary primitives available to compiled El programs.
  * All functions use the __ prefix convention. Signatures use el_val_t (= int64_t)
- * as the universal value type, consistent with el_runtime.h.
+ * as the universal value type.
  *
- * This file is the clean OS boundary for the El runtime migration. All heavy
- * interpreter state, legacy naming, and implicit global coupling lives in
- * el_runtime.c. el_seed.c exposes only the minimal primitives that compiled
- * El programs need to call into the OS.
+ * el_seed.c is the complete C boundary for the El runtime. The heavy runtime
+ * (el_runtime.c) has been retired — everything lives in el_seed.c plus the
+ * native El runtime (runtime/ *.el files).
  *
- * Link requirements (same as el_runtime.c):
+ * Link requirements:
  *   -lcurl    — HTTP client (__http_do, __http_do_to_file)
  *   -lpthread — threading (__thread_create, __thread_join, __mutex_new, ...)
  *
- * Canonical compile:
- *   cc -std=c11 -I el-compiler/runtime -lcurl -lpthread \
- *      -o <out> <prog>.c el-compiler/runtime/el_seed.c el-compiler/runtime/el_runtime.c
+ * Canonical compile (via elb):
+ *   elb builds and links el_seed.c automatically.
  */
 
 #pragma once
 
-#include "el_runtime.h"  /* el_val_t, EL_STR, EL_CSTR, EL_INT, EL_NULL, el_to_float, el_from_float */
+#include <stdint.h>
+#include <stdlib.h>
+
+/* ── Value model ─────────────────────────────────────────────────────────────
+ * All El values are el_val_t (int64_t). On 64-bit systems a pointer fits.
+ *   String  -> el_val_t  (holds const char* via uintptr_t cast)
+ *   Int     -> el_val_t  (stored directly)
+ *   Bool    -> el_val_t  (0 = false, nonzero = true)
+ *   Void    -> void
+ */
+typedef int64_t el_val_t;
+
+#define EL_STR(s)  ((el_val_t)(uintptr_t)(s))
+#define EL_CSTR(v) ((const char*)(uintptr_t)(v))
+#define EL_INT(v)  (v)
+#define EL_NULL    ((el_val_t)0)
+
+/* Float values share the el_val_t slot via bit-cast. */
+static inline double el_to_float(el_val_t v) {
+    union { int64_t i; double f; } u; u.i = (int64_t)v; return u.f;
+}
+static inline el_val_t el_from_float(double f) {
+    union { double f; int64_t i; } u; u.f = f; return (el_val_t)u.i;
+}
 
 #ifdef __cplusplus
 extern "C" {