feat(Phase 9.75j): Complete warning elimination - 106→0 warnings (100% improvement)

✨ Major code quality improvements:
• Fixed all unused variable and parameter warnings
• Added appropriate #[allow(dead_code)] annotations
• Renamed constants to follow Rust naming conventions
• Achieved completely warning-free codebase

🔧 Key changes:
• Parser expressions: Fixed arg_count and statement_count usage
• MIR modules: Added dead_code allows for future-use code
• Backend modules: Prefixed unused parameters with underscore
• Effect constants: Renamed 'read' to 'READ_ALIAS' for conventions

📊 Results:
• Before: 106 warnings (noisy build output)
• After: 0 warnings (clean build)
• Improvement: 100% warning reduction

🎯 This continues the bug fixing initiative "つづきのしゅうせいおねがいにゃ！"
from Phase 9.75i (birth() fixes, static methods, Copilot apps).

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/backend/vm.rs

@@ -129,6 +129,7 @@ pub struct VM {
     /// Program counter within current block
     pc: usize,
     /// Return value from last execution
+    #[allow(dead_code)]
     last_result: Option<VMValue>,
     /// Simple field storage for objects (maps reference -> field -> value)
     object_fields: HashMap<ValueId, HashMap<String, VMValue>>,