M3: Asset pipeline, sprite/palette/background declarations, debug symbols

Parser extensions:
- sprite declarations with chr: @chr("file.png"), @binary("file.bin"), or inline [hex]
- palette declarations with colors: [0x0F, 0x00, 0x10, 0x20]
- background declarations with chr: asset source
- @chr/@binary asset source parsing
- load_background and set_palette statements
- --debug CLI flag (plumbed through, not yet wired to codegen)

Asset pipeline (new module):
- PNG → CHR tile conversion using image crate (8x8 tiles, 2-bitplane encoding)
- NES color palette table (all 64 standard NES colors as RGB)
- Nearest-color matching (Euclidean distance in RGB space)

Debug module (new):
- Source map (ROM address → source Span mapping)
- Debug symbols with variable address table
- Mesen-compatible .mlb label export
- .sym symbol table export

192 tests total (13 new: 5 parser + 3 asset + 5 debug)

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/parser/ast.rs

@@ -7,10 +7,41 @@ pub struct Program {
     pub constants: Vec<ConstDecl>,
     pub functions: Vec<FunDecl>,
     pub states: Vec<StateDecl>,
+    pub sprites: Vec<SpriteDecl>,
+    pub palettes: Vec<PaletteDecl>,
+    pub backgrounds: Vec<BackgroundDecl>,
     pub start_state: String,
     pub span: Span,
 }
 
+#[derive(Debug, Clone)]
+pub struct SpriteDecl {
+    pub name: String,
+    pub chr_source: AssetSource,
+    pub span: Span,
+}
+
+#[derive(Debug, Clone)]
+pub struct PaletteDecl {
+    pub name: String,
+    pub colors: Vec<u8>,
+    pub span: Span,
+}
+
+#[derive(Debug, Clone)]
+pub struct BackgroundDecl {
+    pub name: String,
+    pub chr_source: AssetSource,
+    pub span: Span,
+}
+
+#[derive(Debug, Clone)]
+pub enum AssetSource {
+    Chr(String),
+    Binary(String),
+    Inline(Vec<u8>),
+}
+
 #[derive(Debug, Clone)]
 pub struct GameDecl {
     pub name: String,
@@ -188,6 +219,8 @@ pub enum Statement {
     Transition(String, Span),
     WaitFrame(Span),
     Call(String, Vec<Expr>, Span),
+    LoadBackground(String, Span),
+    SetPalette(String, Span),
 }
 
 #[derive(Debug, Clone)]