Sprite resolution, asset wiring, shift-assign, unreachable state warning

Sprite/asset pipeline:
- Linker::link_with_assets() places sprite CHR data in ROM at correct tile
- assets::resolve_sprites() walks Program for inline sprite bytes
- CodeGen::with_sprites() maps sprite names to tile indices
- gen_draw() uses correct tile index from sprite declarations
- main.rs wires the full resolution pipeline

Shift-assign operators (<<= and >>=):
- AssignOp::ShiftLeftAssign and ShiftRightAssign variants
- Parser handles in both statement and array index contexts
- Codegen emits ASL A / LSR A
- IR lowering maps to ShiftLeft/ShiftRight ops

Unreachable state warning (W0104):
- BFS from start state finds reachable states via transitions
- States not reached produce W0104 warning

Error polish helpers:
- suggest_var_name() for "did you mean" suggestions
- emit_undefined_var() for E0502 with typo hints
- Used by analyzer for better diagnostics

242 tests pass, clippy clean.

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/parser/mod.rs

@@ -963,6 +963,26 @@ impl Parser {
                     start,
                 ))
             }
+            TokenKind::ShiftLeftAssign => {
+                self.advance();
+                let value = self.parse_expr()?;
+                Ok(Statement::Assign(
+                    LValue::Var(name),
+                    AssignOp::ShiftLeftAssign,
+                    value,
+                    start,
+                ))
+            }
+            TokenKind::ShiftRightAssign => {
+                self.advance();
+                let value = self.parse_expr()?;
+                Ok(Statement::Assign(
+                    LValue::Var(name),
+                    AssignOp::ShiftRightAssign,
+                    value,
+                    start,
+                ))
+            }
             TokenKind::LBracket => {
                 // Array index assignment: name[index] = value
                 self.advance();
@@ -1027,6 +1047,14 @@ impl Parser {
                 self.advance();
                 Ok(AssignOp::CaretAssign)
             }
+            TokenKind::ShiftLeftAssign => {
+                self.advance();
+                Ok(AssignOp::ShiftLeftAssign)
+            }
+            TokenKind::ShiftRightAssign => {
+                self.advance();
+                Ok(AssignOp::ShiftRightAssign)
+            }
             _ => Err(Diagnostic::error(
                 ErrorCode::E0201,
                 format!("expected assignment operator, found '{}'", self.peek()),