refactor(phi): Phase 26-B-3 - loop_builder.rs and json_v0_bridge PhiInputCollector integration

- loop_builder.rs: Replace LoopSnapshotMergeBox calls with PhiInputCollector for continue-merge PHI generation
- json_v0_bridge/lowering/loop_.rs: Replace LoopSnapshotMergeBox calls with PhiInputCollector for continue_merge_bb PHI generation
- Unified PHI input handling across all loop builders (loopform_builder, loop_builder, json_v0_bridge)
- Tests: mir_loopform_exit_phi all passed (4/4)

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

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

src/mir/loop_builder.rs

@@ -9,6 +9,7 @@ use super::{BasicBlockId, ConstValue, MirInstruction, ValueId};
 use crate::mir::control_form::{ControlForm, IfShape, LoopShape, is_control_form_trace_on};
 use crate::mir::phi_core::loopform_builder::{LoopFormBuilder, LoopFormOps};
 use crate::mir::phi_core::loop_snapshot_merge::LoopSnapshotMergeBox;
+use crate::mir::phi_core::phi_input_collector::PhiInputCollector;
 use crate::ast::ASTNode;
 use std::collections::HashMap;
 
@@ -431,17 +432,21 @@ impl<'a> LoopBuilder<'a> {
                 }
             }
 
-            // 各変数について PHI ノードを生成（LoopSnapshotMergeBox の最適化を使用）
+            // 各変数について PHI ノードを生成（Phase 26-B-3: PhiInputCollector使用）
             let mut merged = HashMap::new();
-            for (var_name, mut inputs) in all_vars {
+            for (var_name, inputs) in all_vars {
+                // Phase 26-B-3: Use PhiInputCollector
+                let mut collector = PhiInputCollector::new();
+                collector.add_snapshot(&inputs);
+                collector.sanitize();
+
                 // Phase 25.2: optimize_same_value() で最適化判定
-                let result_value = if let Some(same_val) = LoopSnapshotMergeBox::optimize_same_value(&inputs) {
+                let result_value = if let Some(same_val) = collector.optimize_same_value() {
                     // 全て同じ値 or 単一入力 → PHI 不要
                     same_val
                 } else {
                     // 異なる値を持つ場合は PHI ノードを生成
-                    // Phase 25.2: sanitize_inputs() で入力を正規化
-                    LoopSnapshotMergeBox::sanitize_inputs(&mut inputs);
+                    let final_inputs = collector.finalize();
 
                     let phi_id = self.new_value();
 
@@ -449,7 +454,7 @@ impl<'a> LoopBuilder<'a> {
                         if let Some(merge_block) = func.blocks.get_mut(&continue_merge_id) {
                             merge_block.add_instruction(MirInstruction::Phi {
                                 dst: phi_id,
-                                inputs,
+                                inputs: final_inputs,
                             });
                         }
                     }