add DOM bridge, async/await, window export, and native_js to JS target

- el_runtime.js: add 19 dom_* builtins (browser-only, throw in Node),
  window_set/window_get for exposing El functions to the browser global
  scope, and native_js/native_js_call escape hatches for third-party libs
- codegen-js.el: destructure all new builtins in generated preamble; add
  @async decorator support that emits async function + await at call sites
  for known-async HTTP builtins and user-declared @async functions; pre-
  registration pass ensures forward calls to @async functions get await
- spec/codegen-js.md: mark Phase 3 (DOM bridge) implemented, document
  @async approach and its limitations, update builtin table and status
- examples/browser-counter.el: canonical example showing dom_get_element,
  dom_set_text, dom_is_null, window_set, and state_set/get

examples/browser-counter.el

@@ -0,0 +1,41 @@
+// browser-counter.el — canonical browser DOM bridge example
+//
+// Compile with:  elc --target=js examples/browser-counter.el > counter.js
+//
+// Then include in an HTML page that has a <span id="count-display"> element.
+// The page can call window.increment() from any onclick handler, e.g.:
+//   <button onclick="increment()">+1</button>
+//
+// On load the display is initialised to "0".  Each call to increment()
+// adds 1 and updates the display text.
+//
+// Demonstrates:
+//   - dom_get_element   to locate a DOM node by id
+//   - dom_set_text      to update visible text content
+//   - dom_is_null       to guard against missing elements
+//   - window_set        to expose an El function for inline event handlers
+//   - state_set/get     for in-memory counter state (survives calls, resets
+//                       on page reload — same semantics as the C state_* API)
+
+fn init() -> Void {
+    state_set("counter", 0)
+    let display = dom_get_element("count-display")
+    if !dom_is_null(display) {
+        dom_set_text(display, "0")
+    }
+}
+
+fn increment() -> Void {
+    let current = str_to_int(state_get("counter"))
+    let next = current + 1
+    state_set("counter", next)
+    let display = dom_get_element("count-display")
+    if !dom_is_null(display) {
+        dom_set_text(display, int_to_str(next))
+    }
+}
+
+fn main() -> Void {
+    init()
+    window_set("increment", increment)
+}