parser/lowering: declarative metasprites for multi-tile sprite groups

Multi-tile sprites used to require one hand-written `draw` per tile,
e.g. the four-call sequence in `examples/platformer.ne`'s
`draw_player()`. The new `metasprite Name { ... }` declaration
collects parallel `dx`/`dy`/`frame` arrays plus a reference to the
underlying sprite, and `draw Name at: (x, y)` expands to one OAM
slot per tile in the IR lowering — the codegen sees N regular
DrawSprite ops, so the runtime OAM cursor allocator picks them up
without any metasprite-specific awareness.

The metasprite's `frame:` array is interpreted *relative to the
underlying sprite's base tile*: index 0 means "the first tile this
sprite owns", which is the natural reading for a 16×16 hero whose
pixel art the asset resolver split into four consecutive tiles.
The lowering walks `program.sprites` to compute base tile indices
the same way `assets::resolve_sprites` would, then folds the base
into each frame entry before storing the metasprite info. Sprites
sourced from external `@chr(...)` / `@binary(...)` files whose
bytes aren't available at parse time fall back to a one-tile
assumption — those programs are rare and can declare metasprites
against pixel-art sprites instead.

The new `examples/metasprite_demo.ne` declares a 16×16 hero sprite
and arranges its four tiles into a metasprite, then sweeps the
hero across the screen so the harness captures it mid-motion.
The new keyword is added to the lexer/token list, and the parser
accepts `sprite:` (the otherwise-keyword) as a property name in
metasprite bodies so the natural spelling parses.

https://claude.ai/code/session_01KEczoNUX3WmcFLfq6iAQxB

src/ir/tests.rs

@@ -350,6 +350,69 @@ fn lower_debug_frame_overrun_count_emits_peek() {
     );
 }
 
+#[test]
+fn lower_metasprite_draw_expands_to_one_op_per_tile() {
+    // `draw Hero at: (10, 20)` where Hero is a 4-tile metasprite
+    // should lower to four `DrawSprite` ops, each with the
+    // metasprite's underlying sprite name and one tile from the
+    // declaration's `frame:` array (offset by the sprite's base
+    // tile index — the runtime smiley occupies tile 0, so a
+    // single-sprite program starts user tiles at 1).
+    let ir = lower_ok(
+        r#"
+        game "T" { mapper: NROM }
+        sprite Tile {
+            pixels: [
+                "@@@@@@@@",
+                "@@@@@@@@",
+                "@@@@@@@@",
+                "@@@@@@@@",
+                "@@@@@@@@",
+                "@@@@@@@@",
+                "@@@@@@@@",
+                "@@@@@@@@"
+            ]
+        }
+        metasprite Hero {
+            sprite: Tile
+            dx:    [0, 8, 0, 8]
+            dy:    [0, 0, 8, 8]
+            frame: [0, 0, 0, 0]
+        }
+        on frame {
+            draw Hero at: (10, 20)
+            wait_frame
+        }
+        start Main
+    "#,
+    );
+    let frame_fn = ir
+        .functions
+        .iter()
+        .find(|f| f.name.contains("frame"))
+        .unwrap();
+    let draws: Vec<_> = frame_fn
+        .blocks
+        .iter()
+        .flat_map(|b| &b.ops)
+        .filter_map(|op| match op {
+            IrOp::DrawSprite { sprite_name, .. } => Some(sprite_name.clone()),
+            _ => None,
+        })
+        .collect();
+    assert_eq!(
+        draws.len(),
+        4,
+        "metasprite with 4 tiles should expand to 4 DrawSprite ops"
+    );
+    for name in &draws {
+        assert_eq!(
+            name, "Tile",
+            "expanded ops should target the underlying sprite"
+        );
+    }
+}
+
 #[test]
 fn lower_nested_struct_literal_init_expands_to_leaves() {
     // A `Hero { pos: Vec2 { x: 1, y: 2 }, hp: 100, inv: [3,4,5,6] }`