feat(core): unified input state (keyboard/mouse/scroll)

Add InputState (DRAFT Etapes 15, sous-volt 15.B): pressed/held/released
semantics via HashSet rotation, mouse position/delta/scroll accumulators.
Wire it into App (pub field), forward winit WindowEvents in the event
handler, and begin_frame/end_frame around AppHandler::update.

Handleable key/mouse/scroll logic extracted into private helpers so all
five unit tests avoid constructing winit KeyEvent (private fields).

lib/src/core/mod.rs

@@ -11,9 +11,11 @@
 
 pub mod context;
 pub mod frame;
+pub mod input;
 pub mod renderer;
 
 // Re-exports
 pub use context::Context;
 pub use frame::Frame;
+pub use input::InputState;
 pub use renderer::Renderer;