Add HTML template codegen and runtime for JS backend

JS codegen (codegen-js.el):
- js_cg_html_template: emits an IIFE that builds HTML via += concat
- js_cg_html_element_str / js_cg_html_parts / js_cg_html_attrs_str:
  mirror the C codegen structure using JS string accumulator
- js_cg_html_each: {#each} compiles to a JS for-loop
- Reuses existing js_str_lit / js_escape from the file header

Runtime (el_runtime.js):
- html_escape(s): replaces & < > " ' using regex chains
- html_raw(s): identity function
- Both exported from the runtime module exports object

el-compiler/runtime/el_runtime.js

@@ -128,6 +128,22 @@ function str_pad_right(s, width, pad) {
     return String(s).padEnd(width, String(pad));
 }
 
+// ── 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).
+
+function html_escape(s) {
+    return String(s)
+        .replace(/&/g, '&amp;')
+        .replace(/</g, '&lt;')
+        .replace(/>/g, '&gt;')
+        .replace(/"/g, '&quot;')
+        .replace(/'/g, '&#39;');
+}
+
+function html_raw(s) { return s; }
+
 // ── Math ────────────────────────────────────────────────────────────────────
 
 function el_abs(n) { return Math.abs(n); }
@@ -1017,6 +1033,8 @@ export {
     fs_read, fs_write, fs_list,
     json_parse, json_stringify, json_get, json_get_string, json_get_int,
     time_now, time_now_utc, sleep_ms,
+    // HTML template helpers
+    html_escape, html_raw,
     bool_to_str, exit_program, args, env,
     state_set, state_get, state_del, state_keys,
     el_cgi_init,