feat: add stdlib and browser examples

- stdlib/html.lux: Type-safe HTML construction
- stdlib/browser.lux: Browser utilities
- examples/web/: Counter app with DOM manipulation
- examples/counter.lux: Simple counter example

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

stdlib/browser.lux

@@ -0,0 +1,89 @@
+// Browser Module - Effects for browser/DOM interaction
+//
+// This module provides effects for interacting with the browser DOM,
+// local storage, and other browser APIs.
+
+// Opaque Element type (represents a DOM element)
+type Element =
+    | DomElement(Int)
+
+// DOM manipulation effect
+effect Dom {
+    fn getElementById(id: String): Option<Element>
+    fn querySelector(selector: String): Option<Element>
+    fn querySelectorAll(selector: String): List<Element>
+    fn createElement(tag: String): Element
+    fn createTextNode(text: String): Element
+    fn appendChild(parent: Element, child: Element): Unit
+    fn removeChild(parent: Element, child: Element): Unit
+    fn setAttribute(element: Element, name: String, value: String): Unit
+    fn removeAttribute(element: Element, name: String): Unit
+    fn setProperty(element: Element, name: String, value: String): Unit
+    fn setTextContent(element: Element, text: String): Unit
+    fn getBody(): Element
+    fn focus(element: Element): Unit
+    fn blur(element: Element): Unit
+}
+
+// Browser storage effect
+effect Storage {
+    fn getItem(key: String): Option<String>
+    fn setItem(key: String, value: String): Unit
+    fn removeItem(key: String): Unit
+    fn clear(): Unit
+}
+
+// Browser navigation effect
+effect Navigation {
+    fn pushState(url: String): Unit
+    fn replaceState(url: String): Unit
+    fn back(): Unit
+    fn forward(): Unit
+    fn getLocation(): String
+    fn getPathname(): String
+}
+
+// Browser window effect
+effect Window {
+    fn alert(message: String): Unit
+    fn confirm(message: String): Bool
+    fn scrollTo(x: Int, y: Int): Unit
+    fn getInnerWidth(): Int
+    fn getInnerHeight(): Int
+}
+
+// ============================================================================
+// Subscription types for listening to external events
+// ============================================================================
+
+type Sub<M> =
+    | OnAnimationFrame(fn(Float): M)
+    | OnResize(fn(Int, Int): M)
+    | OnKeyPress(fn(String): M)
+    | OnKeyDown(fn(String): M)
+    | OnKeyUp(fn(String): M)
+    | OnMouseMove(fn(Int, Int): M)
+    | OnClick(fn(Int, Int): M)
+    | OnUrlChange(fn(String): M)
+    | Every(Int, fn(Float): M)
+    | NoSub
+
+// Subscription constructors
+fn onAnimationFrame<M>(toMsg: fn(Float): M): Sub<M> =
+    OnAnimationFrame(toMsg)
+
+fn onResize<M>(toMsg: fn(Int, Int): M): Sub<M> =
+    OnResize(toMsg)
+
+fn onKeyPress<M>(toMsg: fn(String): M): Sub<M> =
+    OnKeyPress(toMsg)
+
+fn every<M>(ms: Int, toMsg: fn(Float): M): Sub<M> =
+    Every(ms, toMsg)
+
+fn noSub<M>(): Sub<M> =
+    NoSub
+
+// Combine multiple subscriptions
+fn batch<M>(subs: List<Sub<M>>): List<Sub<M>> =
+    subs