runtime: add dharma-required functions to el_runtime.c and runtime/*.el

Add the following functions that dharma registry calls but were missing
from the El runtime:

el_runtime.c (consumed by the old build system via released SDK):
  - list_len, list_get — aliases for el_list_len/el_list_get (handlers.el)
  - json_array_push — append pre-encoded element to JSON array string
  - now_millis, unix_timestamp_ms, time_now_ms — ms-since-epoch aliases
  - log_info, log_warn — structured stderr log helpers
  - config — reads config from environment (alias for getenv)
  - http_patch — HTTP PATCH with Content-Type: application/json
  - http_post_engram — HTTP POST with optional X-API-Key header
  - http_get_engram — HTTP GET with optional X-API-Key header
  - str_to_bytes — encode string as JSON byte array [72,101,...]
  - bytes_to_str — decode JSON byte array back to string
  - hash_sha256 — SHA-256 hex digest using built-in sha256 impl

runtime/*.el (consumed by the new build system):
  - http.el: http_patch, http_post_engram, http_get_engram
  - time.el: now_millis, unix_timestamp_ms, time_now_ms
  - env.el: config, log_info, log_warn, list_len, list_get
  - json.el: json_array_push, bytes_to_str
  - string.el: str_to_bytes, hash_sha256 (via __sha256_hex seed)

el_seed.h / el_seed.c:
  - __sha256_hex primitive with self-contained SHA-256 implementation

el-compiler/runtime/el_seed.h

@@ -233,6 +233,12 @@ el_val_t  __engram_stats_json(void);
 el_val_t  __engram_list_layers_json(void);
 el_val_t  __engram_compile_layered_json(el_val_t intent, el_val_t depth);
 
+/* ── Cryptographic hashing ────────────────────────────────────────────────── */
+
+/* __sha256_hex — return the SHA-256 hex digest of a string.
+ * The returned string is 64 hex characters (lowercase). */
+el_val_t  __sha256_hex(el_val_t s);
+
 /* ── args init (called from main) ────────────────────────────────────────── */
 /* Store argc/argv for __args_json. Call once at the start of main(). */
 void      el_seed_init_args(int argc, char** argv);