refactor: 大規模なモジュールを分割し、コードの構造を改善

runner, mir/builder, backend/llvm の各モジュールが肥大化していたため、責務ごとにファイルを分割し、見通しを改善するリファクタリングを実施。

### `runner`
- `mod.rs` に集中していたロジックを、以下のモジュールに分割:
    - `tasks.rs`: `nyash.toml` のタスク実行処理
    - `build.rs`: AOTビルドパイプラインの実装
    - `pipeline.rs`: `using` の解決など、パイプライン中のユーティリティ
    - `demos.rs`: デモの実行処理

### `mir/builder`
- `if/else` 文のPHIノード生成ロジックを `stmts.rs` から `phi.rs` へ切り出し。
- `utils.rs` にあったPHI関連のヘルパーも `phi.rs` に集約。
- ASTから自由変数を収集するロジックを `vars.rs` へ切り出し。

### `backend/llvm/compiler/codegen`
- 巨大だった `lower_one_function` 関数を、`function.rs` モジュールとして分離。
- `sanitize_symbol` などのヘルパー関数を `utils.rs` へ移動。

src/mir/builder/phi.rs

@@ -0,0 +1,99 @@
+
+use crate::mir::{MirFunction, ValueId, MirType, MirInstruction, BasicBlockId};
+use std::collections::HashMap;
+use crate::ast::ASTNode;
+use super::MirBuilder;
+
+// PHI-based return type inference helper
+pub(super) fn infer_type_from_phi(
+    function: &MirFunction,
+    ret_val: ValueId,
+    types: &HashMap<ValueId, MirType>,
+) -> Option<MirType> {
+    for (_bid, bb) in function.blocks.iter() {
+        for inst in bb.instructions.iter() {
+            if let MirInstruction::Phi { dst, inputs } = inst {
+                if *dst == ret_val {
+                    let mut it = inputs.iter().filter_map(|(_, v)| types.get(v));
+                    if let Some(first) = it.next() {
+                        if it.all(|mt| mt == first) {
+                            return Some(first.clone());
+                        }
+                    }
+                }
+            }
+        }
+    }
+    None
+}
+
+// Local helper for if-statement analysis (moved from stmts.rs)
+pub(super) fn extract_assigned_var(ast: &ASTNode) -> Option<String> {
+    match ast {
+        ASTNode::Assignment { target, .. } => {
+            if let ASTNode::Variable { name, .. } = target.as_ref() { Some(name.clone()) } else { None }
+        }
+        ASTNode::Program { statements, .. } => statements.last().and_then(|st| extract_assigned_var(st)),
+        ASTNode::If { then_body, else_body, .. } => {
+            // Look into nested if: if both sides assign the same variable, propagate that name upward.
+            let then_prog = ASTNode::Program { statements: then_body.clone(), span: crate::ast::Span::unknown() };
+            let tvar = extract_assigned_var(&then_prog);
+            let evar = else_body.as_ref().and_then(|eb| {
+                let ep = ASTNode::Program { statements: eb.clone(), span: crate::ast::Span::unknown() };
+                extract_assigned_var(&ep)
+            });
+            match (tvar, evar) {
+                (Some(tv), Some(ev)) if tv == ev => Some(tv),
+                _ => None,
+            }
+        }
+        _ => None,
+    }
+}
+
+impl MirBuilder {
+    /// Normalize Phi creation for if/else constructs.
+    /// This handles variable reassignment patterns and ensures a single exit value.
+    pub(super) fn normalize_if_else_phi(
+        &mut self,
+        then_block: BasicBlockId,
+        else_block: BasicBlockId,
+        then_value_raw: ValueId,
+        else_value_raw: ValueId,
+        pre_if_var_map: &HashMap<String, ValueId>,
+        then_ast_for_analysis: &ASTNode,
+        else_ast_for_analysis: &Option<ASTNode>,
+        then_var_map_end: &HashMap<String, ValueId>,
+        else_var_map_end_opt: &Option<HashMap<String, ValueId>>,
+        pre_then_var_value: Option<ValueId>,
+    ) -> Result<ValueId, String> {
+        // If only the then-branch assigns a variable (e.g., `if c { x = ... }`) and the else
+        // does not assign the same variable, bind that variable to a Phi of (then_value, pre_if_value).
+        let assigned_var_then = extract_assigned_var(then_ast_for_analysis);
+        let assigned_var_else = else_ast_for_analysis.as_ref().and_then(|a| extract_assigned_var(a));
+        let result_val = self.value_gen.next();
+
+        if let Some(var_name) = assigned_var_then.clone() {
+            let else_assigns_same = assigned_var_else.as_ref().map(|s| s == &var_name).unwrap_or(false);
+            // Resolve branch-end values for the assigned variable
+            let then_value_for_var = then_var_map_end.get(&var_name).copied().unwrap_or(then_value_raw);
+            let else_value_for_var = if else_assigns_same {
+                else_var_map_end_opt.as_ref().and_then(|m| m.get(&var_name).copied()).unwrap_or(else_value_raw)
+            } else {
+                // Else doesn't assign: use pre-if value if available
+                pre_then_var_value.unwrap_or(else_value_raw)
+            };
+            // Emit Phi for the assigned variable and bind it
+            self.emit_instruction(MirInstruction::Phi { dst: result_val, inputs: vec![(then_block, then_value_for_var), (else_block, else_value_for_var)] })?;
+            self.variable_map = pre_if_var_map.clone();
+            self.variable_map.insert(var_name, result_val);
+        } else {
+            // No variable assignment pattern detected – just emit Phi for expression result
+            self.emit_instruction(MirInstruction::Phi { dst: result_val, inputs: vec![(then_block, then_value_raw), (else_block, else_value_raw)] })?;
+            // Merge variable map conservatively to pre-if snapshot (no new bindings)
+            self.variable_map = pre_if_var_map.clone();
+        }
+
+        Ok(result_val)
+    }
+}