runtime: rename str_format param 'template' to 'fmt'

template is a reserved keyword in C++; though not in C, it blocks
this header from ever being included from C++ code. Match printf-
family convention with fmt instead.

The deeper question of whether string-template substitution is the
right abstraction for our substrate is filed separately as backlog.

el-compiler/runtime/el_runtime.h

@@ -199,6 +199,19 @@ el_val_t  http_get_to_file(el_val_t url, el_val_t headers_map, el_val_t output_p
 el_val_t  url_encode(el_val_t s);   /* RFC 3986 unreserved set */
 el_val_t  url_decode(el_val_t s);   /* '+' → space, %XX → byte  */
 
+/* ── HTML allowlist sanitizer ────────────────────────────────────────────────
+ * el_html_sanitize(input_html, allowlist_json) — strict allowlist HTML
+ * cleaner. State-machine parser; tag/attribute names compared case-
+ * insensitively against the allowlist; `<a href>` / `<… src>` URL schemes
+ * validated (http, https, mailto, fragment-only, or relative); whole-
+ * subtree drop for script / style / iframe / object / embed / form; HTML-
+ * escapes free text outside dropped subtrees.
+ *
+ * The allowlist is JSON of the form
+ *     {"p":[],"a":["href","title"],"strong":[],...}
+ * where each value is the array of attribute names allowed for that tag. */
+el_val_t  el_html_sanitize(el_val_t input_html, el_val_t allowlist_json);
+
 /* ── Filesystem ──────────────────────────────────────────────────────────── */
 
 el_val_t  fs_read(el_val_t path);