codegen: emit C instead of bytecode — El is now natively compiled

Rewrites codegen.el to produce C source instead of JSON bytecode,
eliminating the ELVM interpreter as a runtime dependency.

- All El values use el_val_t (int64_t) as the universal type; integers
  are stored directly, strings/pointers via uintptr_t cast
- String literals wrapped with EL_STR(), arithmetic works natively
- fn declarations become C functions returning el_val_t
- let bindings become el_val_t local variables
- if/else, while, for all map to native C control flow
- String + String uses el_str_concat(); numeric + uses C +
- strip_outer_parens() prevents double-paren warnings in if/while
- compiler.el updated to describe C output and correct CLI usage

Adds el-compiler/runtime/ with:
- el_runtime.h: declares all builtins using el_val_t
- el_runtime.c: implements I/O, strings, math, list, map, fs, JSON;
  HTTP builtins are stubs (return empty string) pending libcurl

Compile El programs with:
  cc -I<runtime-dir> -o hello hello.c el_runtime.c

el-compiler/runtime/el_runtime.h

@@ -0,0 +1,108 @@
+/*
+ * el_runtime.h — El language C runtime header
+ *
+ * Declares all built-in functions available to compiled El programs.
+ * Include this in every generated .c file.
+ *
+ * Value model:
+ *   All El values are represented as el_val_t (= int64_t).
+ *   On 64-bit systems a pointer fits in int64_t.
+ *   String values are cast: (el_val_t)(uintptr_t)"hello"
+ *   Integer values are stored directly.
+ *   This lets arithmetic work naturally while still passing strings around.
+ *
+ * Type conventions (El -> C):
+ *   String  -> el_val_t  (holds const char* via uintptr_t cast)
+ *   Int     -> el_val_t
+ *   Bool    -> el_val_t  (0 = false, nonzero = true)
+ *   Any     -> el_val_t
+ *   Void    -> void
+ *
+ * Macros for convenience:
+ *   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
+ */
+
+#pragma once
+
+#include <stdint.h>
+#include <stdlib.h>
+
+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)
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* ── I/O ──────────────────────────────────────────────────────────────────── */
+
+void      println(el_val_t s);
+void      print(el_val_t s);
+el_val_t  readline(void);
+
+/* ── String builtins ─────────────────────────────────────────────────────── */
+
+el_val_t  el_str_concat(el_val_t a, el_val_t b);
+el_val_t  str_eq(el_val_t a, el_val_t b);
+el_val_t  str_starts_with(el_val_t s, el_val_t prefix);
+el_val_t  str_ends_with(el_val_t s, el_val_t suffix);
+el_val_t  str_len(el_val_t s);
+el_val_t  str_concat(el_val_t a, el_val_t b);
+el_val_t  int_to_str(el_val_t n);
+el_val_t  str_to_int(el_val_t s);
+el_val_t  str_slice(el_val_t s, el_val_t start, el_val_t end);
+el_val_t  str_contains(el_val_t s, el_val_t sub);
+el_val_t  str_replace(el_val_t s, el_val_t from, el_val_t to);
+el_val_t  str_to_upper(el_val_t s);
+el_val_t  str_to_lower(el_val_t s);
+el_val_t  str_trim(el_val_t s);
+
+/* ── Math ────────────────────────────────────────────────────────────────── */
+
+el_val_t  el_abs(el_val_t n);
+el_val_t  el_max(el_val_t a, el_val_t b);
+el_val_t  el_min(el_val_t a, el_val_t b);
+
+/* ── List ────────────────────────────────────────────────────────────────── */
+
+el_val_t  el_list_new(el_val_t count, ...);
+el_val_t  el_list_len(el_val_t list);
+el_val_t  el_list_get(el_val_t list, el_val_t index);
+el_val_t  el_list_append(el_val_t list, el_val_t elem);
+el_val_t  el_list_empty(void);
+
+/* ── Map ─────────────────────────────────────────────────────────────────── */
+
+el_val_t  el_map_new(el_val_t pair_count, ...);
+el_val_t  el_get_field(el_val_t map, el_val_t key);
+el_val_t  el_map_get(el_val_t map, el_val_t key);
+el_val_t  el_map_set(el_val_t map, el_val_t key, el_val_t value);
+
+/* ── HTTP ─────────────────────────────────────────────────────────────────── */
+
+el_val_t  http_get(el_val_t url);
+el_val_t  http_post(el_val_t url, el_val_t body);
+void      http_serve(el_val_t port, el_val_t handler);
+
+/* ── Filesystem ──────────────────────────────────────────────────────────── */
+
+el_val_t  fs_read(el_val_t path);
+el_val_t  fs_write(el_val_t path, el_val_t content);
+
+/* ── JSON ────────────────────────────────────────────────────────────────── */
+
+el_val_t  json_get(el_val_t json, el_val_t key);
+
+/* ── Process ─────────────────────────────────────────────────────────────── */
+
+void      exit_program(el_val_t code);
+
+#ifdef __cplusplus
+}
+#endif