Add debug.log and debug.assert statements

Parser:
- debug.log(expr, ...) and debug.assert(cond) syntax
- parse_debug_statement() handles both methods
- New AST variants: Statement::DebugLog and Statement::DebugAssert

Codegen:
- CodeGen::with_debug(bool) to toggle debug instrumentation
- DebugLog writes each expression to $4800 (Mesen debug port)
- DebugAssert evaluates condition; on false, writes $FF to $4800 + BRK
- Both are stripped entirely when debug_mode = false (release builds)

CLI:
- --debug flag now wired through to CodeGen
- Without --debug, all debug statements produce zero bytes

Analyzer:
- Type-checks DebugAssert condition as bool
- Walks DebugLog args for reads (used_vars tracking)

IR lowering:
- DebugLog/DebugAssert are no-ops (codegen handles them directly)

254 tests (5 new: 3 parser + 2 codegen).

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/main.rs

@@ -80,7 +80,7 @@ fn dump_asm(instructions: &[nescript::asm::Instruction]) {
     }
 }
 
-fn compile(input: &PathBuf, _debug: bool, asm_dump: bool) -> Result<Vec<u8>, ()> {
+fn compile(input: &PathBuf, debug: bool, asm_dump: bool) -> Result<Vec<u8>, ()> {
     let raw_source = std::fs::read_to_string(input).map_err(|e| {
         eprintln!("error: failed to read {}: {e}", input.display());
     })?;
@@ -130,8 +130,9 @@ fn compile(input: &PathBuf, _debug: bool, asm_dump: bool) -> Result<Vec<u8>, ()>
     })?;
 
     // Code generation (still AST-based for M2; IR codegen comes in M3)
-    let codegen =
-        CodeGen::new(&analysis.var_allocations, &program.constants).with_sprites(&sprites);
+    let codegen = CodeGen::new(&analysis.var_allocations, &program.constants)
+        .with_sprites(&sprites)
+        .with_debug(debug);
     let instructions = codegen.generate(&program);
 
     if asm_dump {