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

src/lib.rs

@@ -0,0 +1,13 @@
+pub mod signal;
+pub mod query;
+pub mod context;
+pub mod connection;
+pub mod sse;
+pub mod server;
+
+pub use signal::{Signal, SignalStore, SignalOpts, mangle, client_only, global, data_signals};
+pub use query::{CacheKey, Query};
+pub use context::{RenderContext, ComponentTree, ComponentNode};
+pub use connection::{ConnectionManager, ConnectionState, SubscriptionRegistry};
+pub use sse::{format_patch_elements, format_patch_signals};
+pub use server::{AppState, SharedState, sse_handler};