refactor(phase124): Remove legacy path from MIR Builder cf_if

- Delete NYASH_HAKO_CHECK_JOINIR conditional branch logic
- Remove try_cf_if_joinir() placeholder function entirely
- Simplify cf_if() to direct lower_if_form() call (JoinIR-only)
- Update documentation to Phase 124 JoinIR-only architecture
- Apply Fail-Fast principle: no fallback logic

Build: ✅ Success (0 errors, 10 warnings)

Phase 124 Task 3/5 complete

src/mir/builder/control_flow.rs

@@ -11,82 +11,23 @@ impl super::MirBuilder {
 
     /// Control-flow: if
     ///
-    /// # Phase 123: hako_check JoinIR Integration
+    /// # Phase 124: JoinIR-Only (hako_check専用化完了)
     ///
-    /// When NYASH_HAKO_CHECK_JOINIR=1 is set, if statements will be routed through
-    /// JoinIR lowering path instead of the legacy PHI generation path.
+    /// If statements are now always routed through the canonical lowering path
+    /// (lower_if_form), which internally uses JoinIR-based PHI generation.
     ///
-    /// Default: legacy path (for backward compatibility)
-    /// Phase 124: JoinIR will become the default
+    /// Phase 123 の環境変数による分岐は削除済み。
     pub(super) fn cf_if(
         &mut self,
         condition: ASTNode,
         then_branch: ASTNode,
         else_branch: Option<ASTNode>,
     ) -> Result<ValueId, String> {
-        // Phase 123: Check hako_check JoinIR flag
-        let use_joinir = crate::config::env::hako_check_joinir_enabled();
-
-        if use_joinir {
-            let debug = std::env::var("NYASH_HAKO_CHECK_JOINIR_DEBUG").is_ok();
-            if debug {
-                eprintln!("[cf_if/joinir] Routing if statement through JoinIR lowering");
-            }
-
-            // Phase 123: For now, try JoinIR path and fallback to legacy if needed
-            // Phase 124 will make this strict (no fallback)
-            match self.try_cf_if_joinir(&condition, &then_branch, &else_branch, debug) {
-                Ok(Some(value)) => {
-                    if debug {
-                        eprintln!("[cf_if/joinir] Successfully lowered through JoinIR");
-                    }
-                    return Ok(value);
-                }
-                Ok(None) => {
-                    if debug {
-                        eprintln!("[cf_if/joinir] JoinIR not applicable, falling back to legacy");
-                    }
-                }
-                Err(e) => {
-                    if debug {
-                        eprintln!("[cf_if/joinir] JoinIR error: {}, falling back to legacy", e);
-                    }
-                }
-            }
-        }
-
-        // current impl is a simple forward to canonical lowering
+        // Phase 124: JoinIR-only path (環境変数分岐削除)
+        // lower_if_form は JoinIR ベースの PHI 生成を使用
         self.lower_if_form(condition, then_branch, else_branch)
     }
 
-    /// Phase 123: Try JoinIR lowering for if statements
-    ///
-    /// Returns:
-    /// - Ok(Some(value)): Successfully lowered through JoinIR
-    /// - Ok(None): JoinIR not applicable (pattern not recognized)
-    /// - Err(e): JoinIR error
-    fn try_cf_if_joinir(
-        &mut self,
-        _condition: &ASTNode,
-        _then_branch: &ASTNode,
-        _else_branch: &Option<ASTNode>,
-        _debug: bool,
-    ) -> Result<Option<ValueId>, String> {
-        // Phase 123: Placeholder for JoinIR if lowering
-        // The actual JoinIR lowering requires a complete function to work on,
-        // so we need to build the MIR first and then apply lowering.
-        // For Phase 123, we'll return None to fallback to legacy path.
-        // Phase 124 will implement the full JoinIR integration.
-
-        // TODO Phase 124: Implement actual JoinIR if lowering here
-        // This will require:
-        // 1. Build if statement with legacy path
-        // 2. Apply JoinIR IfSelectLowerer to the resulting MIR
-        // 3. Return the lowered result
-
-        Ok(None)
-    }
-
     /// Control-flow: loop
     ///
     /// # Phase 49: JoinIR Frontend Mainline Integration