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/parser/mod.rs

@@ -163,7 +163,7 @@ impl NyashParser {
     /// プログラム全体をパース
     fn parse_program(&mut self) -> Result<ASTNode, ParseError> {
         let mut statements = Vec::new();
-        let mut statement_count = 0;
+        let mut _statement_count = 0;
         
         while !self.is_at_end() {
             
@@ -180,7 +180,7 @@ impl NyashParser {
             
             let statement = self.parse_statement()?;
             statements.push(statement);
-            statement_count += 1;
+            _statement_count += 1;
         }