feat: Verbatim MVP - terminal renderer, cellular automaton, Verlet physics

- World: 250x250 grid with 14 materials (sand, water, lava, stone, wood, etc.)
- Physics: cellular automaton for materials + Verlet solver for entities
- Entity: multi-cell humanoid (7 sub-bodies with distance constraints)
- Render: terminal renderer with ANSI colors and diff-based updates
- Game loop: fixed 60Hz timestep with accumulator pattern
- Input: WASD movement, number keys for material painting

src/input.rs

@@ -0,0 +1,112 @@
+use crossterm::event::{self, Event, KeyCode, KeyEvent, KeyModifiers};
+use std::time::Duration;
+
+pub enum Action {
+    MoveLeft,
+    MoveRight,
+    Jump,
+    MoveCameraUp,
+    MoveCameraDown,
+    MoveCameraLeft,
+    MoveCameraRight,
+    Paint(MaterialBrush),
+    Quit,
+    None,
+}
+
+pub enum MaterialBrush {
+    Sand,
+    Water,
+    Stone,
+    Lava,
+    Wood,
+    Acid,
+    Grass,
+    Dirt,
+    Fire,
+    Flesh,
+    Erase,
+}
+
+pub struct InputHandler {
+    pub paint_brush: MaterialBrush,
+}
+
+impl InputHandler {
+    pub fn new() -> Self {
+        Self {
+            paint_brush: MaterialBrush::Sand,
+        }
+    }
+
+    pub fn poll(&mut self) -> Action {
+        if !event::poll(Duration::from_millis(0)).unwrap_or(false) {
+            return Action::None;
+        }
+        match event::read() {
+            Ok(Event::Key(KeyEvent { code, modifiers, .. })) => {
+                if modifiers.contains(KeyModifiers::CONTROL) && code == KeyCode::Char('c') {
+                    return Action::Quit;
+                }
+                match code {
+                    KeyCode::Char('q') => Action::Quit,
+                    KeyCode::Left | KeyCode::Char('a') => Action::MoveLeft,
+                    KeyCode::Right | KeyCode::Char('d') => Action::MoveRight,
+                    KeyCode::Up | KeyCode::Char('w') | KeyCode::Char(' ') => Action::Jump,
+                    KeyCode::Char('h') => Action::MoveCameraLeft,
+                    KeyCode::Char('l') => Action::MoveCameraRight,
+                    KeyCode::Char('k') => Action::MoveCameraUp,
+                    KeyCode::Char('j') => Action::MoveCameraDown,
+                    KeyCode::Char('1') => { self.paint_brush = MaterialBrush::Sand; Action::Paint(MaterialBrush::Sand) }
+                    KeyCode::Char('2') => { self.paint_brush = MaterialBrush::Water; Action::Paint(MaterialBrush::Water) }
+                    KeyCode::Char('3') => { self.paint_brush = MaterialBrush::Stone; Action::Paint(MaterialBrush::Stone) }
+                    KeyCode::Char('4') => { self.paint_brush = MaterialBrush::Lava; Action::Paint(MaterialBrush::Lava) }
+                    KeyCode::Char('5') => { self.paint_brush = MaterialBrush::Wood; Action::Paint(MaterialBrush::Wood) }
+                    KeyCode::Char('6') => { self.paint_brush = MaterialBrush::Acid; Action::Paint(MaterialBrush::Acid) }
+                    KeyCode::Char('7') => { self.paint_brush = MaterialBrush::Grass; Action::Paint(MaterialBrush::Grass) }
+                    KeyCode::Char('8') => { self.paint_brush = MaterialBrush::Dirt; Action::Paint(MaterialBrush::Dirt) }
+                    KeyCode::Char('9') => { self.paint_brush = MaterialBrush::Fire; Action::Paint(MaterialBrush::Fire) }
+                    KeyCode::Char('0') => { self.paint_brush = MaterialBrush::Flesh; Action::Paint(MaterialBrush::Flesh) }
+                    KeyCode::Char('x') => { self.paint_brush = MaterialBrush::Erase; Action::Paint(MaterialBrush::Erase) }
+                    _ => Action::None,
+                }
+            }
+            _ => Action::None,
+        }
+    }
+}
+
+impl MaterialBrush {
+    pub fn to_material(&self) -> Option<crate::world::cell::MaterialId> {
+        use crate::world::cell::MaterialId;
+        match self {
+            MaterialBrush::Sand => Some(MaterialId::Sand),
+            MaterialBrush::Water => Some(MaterialId::Water),
+            MaterialBrush::Stone => Some(MaterialId::Stone),
+            MaterialBrush::Lava => Some(MaterialId::Lava),
+            MaterialBrush::Wood => Some(MaterialId::Wood),
+            MaterialBrush::Acid => Some(MaterialId::Acid),
+            MaterialBrush::Grass => Some(MaterialId::Grass),
+            MaterialBrush::Dirt => Some(MaterialId::Dirt),
+            MaterialBrush::Fire => Some(MaterialId::Fire),
+            MaterialBrush::Flesh => Some(MaterialId::Flesh),
+            MaterialBrush::Erase => None,
+        }
+    }
+
+    pub fn name(&self) -> &'static str {
+        match self {
+            MaterialBrush::Sand => "Sand",
+            MaterialBrush::Water => "Water",
+            MaterialBrush::Stone => "Stone",
+            MaterialBrush::Lava => "Lava",
+            MaterialBrush::Wood => "Wood",
+            MaterialBrush::Acid => "Acid",
+            MaterialBrush::Grass => "Grass",
+            MaterialBrush::Dirt => "Dirt",
+            MaterialBrush::Fire => "Fire",
+            MaterialBrush::Flesh => "Flesh",
+            MaterialBrush::Erase => "Erase",
+        }
+    }
+}