Add HTML template codegen and runtime for C backend

C codegen (codegen.el):
- cg_html_template: emits a GCC/Clang statement-expression that
  builds the HTML string via el_str_concat chains
- cg_html_element_str / cg_html_parts / cg_html_attrs_str: recursive
  element and attribute emitters
- cg_html_each: {#each} compiles to a C for-loop with el_list_get
- __html_counter state tracks unique accumulator variable names
- Handles both 'static' (raw string) and 'dynamic' (expr node) attrs
  matching the parser's attribute kind convention

Runtime (el_runtime.c / el_runtime.h):
- html_escape(s): escapes & < > " ' for safe interpolation
- html_raw(s): identity function for raw() bypass
- Both use the existing html_buf_t infrastructure from el_html_sanitize

el-compiler/runtime/el_runtime.h

@@ -214,6 +214,13 @@ el_val_t  url_decode(el_val_t s);   /* '+' → space, %XX → byte  */
  * 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);
 
+/* ── HTML template helpers ───────────────────────────────────────────────────
+ * Used by compiled El HTML template expressions.
+ * html_escape(s) — escape & < > " ' for safe inline interpolation.
+ * html_raw(s)    — identity; explicit opt-out from escaping (`raw()` form). */
+el_val_t  html_escape(el_val_t s);
+el_val_t  html_raw(el_val_t s);
+
 /* ── Filesystem ──────────────────────────────────────────────────────────── */
 
 el_val_t  fs_read(el_val_t path);