Add example games and quick-start guide

Two example .ne programs with compiled .nes ROMs:
- hello_sprite: d-pad controlled sprite movement
- bouncing_ball: auto-bouncing sprite with edge detection

Includes README with build instructions and emulator setup.

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

examples/README.md

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+# NEScript Examples
+
+## Quick Start
+
+### 1. Build the compiler
+
+```
+cargo build --release
+```
+
+### 2. Compile an example
+
+```
+cargo run -- build examples/hello_sprite.ne
+```
+
+This produces `examples/hello_sprite.nes` — a valid iNES ROM file.
+
+### 3. Run in an emulator
+
+Open the `.nes` file in any NES emulator:
+
+- **[Mesen](https://www.mesen.ca/)** (recommended — best debugging support)
+- **[FCEUX](https://fceux.com/)**
+- **[Nestopia](http://nestopia.sourceforge.net/)**
+
+Use the d-pad (arrow keys in most emulators) to move the sprite.
+
+## Examples
+
+| File | Description |
+|------|-------------|
+| `hello_sprite.ne` | Move a smiley face with the d-pad |
+| `bouncing_ball.ne` | A sprite that bounces around the screen automatically |
+
+## What you'll see
+
+The ROM displays a small smiley-face sprite (an 8x8 tile built into the
+compiler's default CHR data). In `hello_sprite`, you control it with the d-pad.
+In `bouncing_ball`, it moves on its own and bounces off the screen edges.
+
+## Emulator controls
+
+| NES Button | Typical Key |
+|------------|-------------|
+| D-pad      | Arrow keys  |
+| A          | Z           |
+| B          | X           |
+| Start      | Enter       |
+| Select     | Right Shift |
+
+(Exact mappings vary by emulator — check your emulator's input settings.)
+
+## Compiler commands
+
+```
+# Compile to ROM
+cargo run -- build examples/hello_sprite.ne
+
+# Compile with custom output path
+cargo run -- build examples/hello_sprite.ne --output my_game.nes
+
+# Type-check only (no ROM output)
+cargo run -- check examples/hello_sprite.ne
+```

examples/bouncing_ball.ne

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+// Bouncing Ball — a sprite that bounces around the screen automatically.
+//
+// Build:  cargo run -- build examples/bouncing_ball.ne
+// Output: examples/bouncing_ball.nes
+
+game "Bouncing Ball" {
+    mapper: NROM
+}
+
+var px: u8 = 64
+var py: u8 = 64
+var dx: u8 = 1     // 1 = moving right, 0 = moving left
+var dy: u8 = 1     // 1 = moving down,  0 = moving up
+
+on frame {
+    // Move horizontally
+    if dx == 1 {
+        px += 1
+        if px >= 240 { dx = 0 }
+    } else {
+        px -= 1
+        if px == 0 { dx = 1 }
+    }
+
+    // Move vertically
+    if dy == 1 {
+        py += 1
+        if py >= 224 { dy = 0 }
+    } else {
+        py -= 1
+        if py == 0 { dy = 1 }
+    }
+
+    draw Ball at: (px, py)
+}
+
+start Main

examples/hello_sprite.ne

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+// Hello Sprite — the simplest NEScript program.
+//
+// Displays a smiley-face sprite on screen and moves it with the d-pad.
+// Build:  cargo run -- build examples/hello_sprite.ne
+// Output: examples/hello_sprite.nes (open in any NES emulator)
+
+game "Hello Sprite" {
+    mapper: NROM
+}
+
+var px: u8 = 128
+var py: u8 = 120
+
+on frame {
+    if button.right { px += 2 }
+    if button.left  { px -= 2 }
+    if button.down  { py += 2 }
+    if button.up    { py -= 2 }
+
+    draw Smiley at: (px, py)
+}
+
+start Main