phase-20.45: PRIMARY no-fallback reps + MIR v0 shape fixes

- Fix MIR v0 shape in lowers: functions[] + name="main" + blocks.id
  * lower_return_int_box.hako
  * lower_return_binop_box.hako
- runner_min: adopt LowerReturnBinOpBox before ReturnInt
- Add PRIMARY no-fallback canaries (all PASS):
  * return-binop / array-size / load-store / return-logical (OR)
- Fix phase2043 runner_min canary alias (Runner -> BuilderRunnerMinBox)
- Update docs: phase-20.45 README (PRIMARY reps), CURRENT_TASK progress

Ancillary: keep builder/provider/canary files in sync; no unrelated behavior changes.

src/mir/builder/exprs_peek.rs

@@ -46,11 +46,12 @@ impl super::MirBuilder {
             phi_inputs.push((else_block, else_val));
             crate::mir::builder::emission::branch::emit_jump(self, merge_block)?;
             self.start_new_block(merge_block)?;
-            // フェーズM: 常にPHI命令を使用（no_phi_mode撤廃）
-            self.emit_instruction(super::MirInstruction::Phi {
-                dst: result_val,
-                inputs: phi_inputs,
-            })?;
+            // フェーズM: PHI はブロック先頭に配置（cf_common 統一）
+            if let (Some(func), Some(cur_bb)) = (self.current_function.as_mut(), self.current_block) {
+                crate::mir::ssot::cf_common::insert_phi_at_head(func, cur_bb, result_val, phi_inputs);
+            } else {
+                self.emit_instruction(super::MirInstruction::Phi { dst: result_val, inputs: phi_inputs })?;
+            }
             return Ok(result_val);
         }
 
@@ -101,11 +102,12 @@ impl super::MirBuilder {
 
         // Merge and yield result
         self.start_new_block(merge_block)?;
-        // フェーズM: 常にPHI命令を使用（no_phi_mode撤廃）
-        self.emit_instruction(super::MirInstruction::Phi {
-            dst: result_val,
-            inputs: phi_inputs,
-        })?;
+        // フェーズM: PHI はブロック先頭に配置（cf_common 統一）
+        if let (Some(func), Some(cur_bb)) = (self.current_function.as_mut(), self.current_block) {
+            crate::mir::ssot::cf_common::insert_phi_at_head(func, cur_bb, result_val, phi_inputs);
+        } else {
+            self.emit_instruction(super::MirInstruction::Phi { dst: result_val, inputs: phi_inputs })?;
+        }
         Ok(result_val)
     }
 }