implement el-ui SSR engine with renderToString and hydration markers

Adds runtime/src/ssr.js: a Node.js SSR engine that parses .el component
source and renders to HTML strings without a DOM. Covers the full template
AST -- elements, text, interpolations, {#if}, {#each}, {#activate}, and
child component recursion.

Every component root element receives hydration markers:
  data-el-component, data-el-id, data-el-state

Event bindings (on:click etc.) are preserved as data-el-* attributes so
the client hydration pass can bind them without re-rendering.

Updates Component base class with:
  - initialState constructor param for SSR state recovery
  - _hydrate(root): binds events on existing DOM, no re-render
  - onHydrate() lifecycle hook

Updates dist/el-ui.js bundle to match.

dist/el-ui.js

@@ -351,15 +351,16 @@ export class Router {
     }
 }
 
-// ── Component base class & mount() ───────────────────────────────────────────
+// ── Component base class, mount(), and hydrate() ─────────────────────────────
 
 export class Component {
-    constructor(props = {}) {
+    constructor(props = {}, initialState = {}) {
         this._graph = new Graph();
         this._stateNodes = {};
         this._state = {};
         this.props = props;
         this._renderer = null;
+        this._ssrState = initialState;
     }
 
     setState(name, value) {
@@ -382,6 +383,20 @@ export class Component {
 
     render() { return ''; }
     onMount() {}
+    onHydrate() {}
+
+    _hydrate(root) {
+        for (const [key, nodeId] of Object.entries(this._stateNodes)) {
+            this._graph.subscribe(nodeId, (node) => {
+                this._state[key] = node.content;
+                if (this._renderer) this._renderer.patch();
+            });
+        }
+        this._renderer = new Renderer(root, this);
+        this._renderer._currentHtml = root.innerHTML;
+        this._renderer._bindEvents();
+        if (typeof this.onHydrate === 'function') this.onHydrate();
+    }
 }
 
 export function mount(ComponentClass, selector, props = {}) {
@@ -394,4 +409,21 @@ export function mount(ComponentClass, selector, props = {}) {
     return component;
 }
 
-export default { mount, Component, Graph, Renderer, Router };
+export function hydrate(selector, ComponentClass, props = {}) {
+    const el = document.querySelector(selector);
+    if (!el) throw new Error(`el-ui hydrate: element not found: ${selector}`);
+    const stateJson = el.getAttribute('data-el-state');
+    const initialState = stateJson ? JSON.parse(stateJson) : {};
+    const instance = new ComponentClass(props, initialState);
+    for (const [key, value] of Object.entries(initialState)) {
+        if (instance._stateNodes[key] !== undefined) {
+            const node = instance._graph.get(instance._stateNodes[key]);
+            if (node) node.content = value;
+            instance._state[key] = value;
+        }
+    }
+    instance._hydrate(el);
+    return instance;
+}
+
+export default { mount, hydrate, Component, Graph, Renderer, Router };