Refactor: untangle framework library from todo example

The codebase is now split into two clear layers:

Framework library (src/):
- lib.rs — public API surface, re-exports all framework types
- signal.rs — SignalOpts, Signal, SignalStore, mangle/global/client_only helpers
- query.rs — Query trait, CacheKey, query_one!/query_all! macros
  (macro paths updated from $crate::framework::* to $crate::*)
- context.rs — RenderContext, ComponentTree, ComponentNode
- connection.rs — ConnectionManager, SubscriptionRegistry, ConnectionState
- sse.rs — format_patch_elements, format_patch_signals
- server.rs — AppState, SharedState, sse_handler (pure framework plumbing)

Todo example (examples/todo/):
- main.rs — SQLite setup, seeding, server bootstrap
- app.rs — router, document_shell, resolve_and_render, render_page,
  action_then_render, and all HTTP handlers
- queries.rs — Todo model, ListTodos/GetTodo queries, mutation helpers
- todo_list.rs — TodoList page component
- about.rs — About page component

Run the example independently with: cargo run --example todo

https://claude.ai/code/session_01JThiQbp3Bn9J1VhBpRuz4u

examples/todo/about.rs

@@ -0,0 +1,47 @@
+use livevue_rs::RenderContext;
+use maud::{html, Markup};
+
+/// Second page component: a simple "About" page.
+///
+/// This exists specifically to exercise:
+/// - The `tree` signal page routing (resolve_root match arm)
+/// - SPA-like navigation via tree signal mutation
+/// - Round-trip: TodoList → About → TodoList without full page reload
+///
+/// In a real app this might show user info, settings, etc.
+pub async fn about(_cx: &mut RenderContext) -> anyhow::Result<Markup> {
+    Ok(html! {
+        div.about-page {
+            h1 { "About LiveVue.rs" }
+
+            nav {
+                button.nav-link
+                    data-on-click="$tree = {page: 'TodoList'}; @post('/render')" {
+                    "Todos"
+                }
+                " | "
+                span.nav-current { "About" }
+            }
+
+            div.about-content {
+                p {
+                    "LiveVue.rs is a reactive server-side rendering framework for Rust. "
+                    "The server renders full-page HTML on every state change. "
+                    "DataStar morphs it into the DOM."
+                }
+                p {
+                    "The architectural bet: "
+                    strong { "full rerender is cheap" }
+                    ", so skip diffing, skip partial updates, skip client-side state management."
+                }
+                h2 { "v0 Status" }
+                ul {
+                    li { "Single process, single node" }
+                    li { "SQLite for storage" }
+                    li { "Subscription keys collected (Phase 2 seam)" }
+                    li { "No cache, no auth, no WAL consumer" }
+                }
+            }
+        }
+    })
+}