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/runner/json_v0_bridge/lowering/loop_.rs

@@ -24,6 +24,7 @@ use super::{lower_stmt_list_with_vars, new_block, BridgeEnv, LoopContext};
 use crate::mir::{BasicBlockId, MirFunction, MirInstruction, ValueId};
 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 std::collections::HashMap;
 use super::super::ast::StmtV0;
 use super::super::ast::ExprV0;
@@ -337,22 +338,25 @@ pub(super) fn lower_loop_stmt(
             }
 
             // 6-2) continue_merge_bb に必要な PHI を生成しつつ、merged_snapshot を作る
+            // Phase 26-B-3: Use PhiInputCollector
             let mut merged_snapshot: HashMap<String, ValueId> = HashMap::new();
-            for (name, mut inputs) in all_inputs {
-                let value = if let Some(same_val) =
-                    LoopSnapshotMergeBox::optimize_same_value(&inputs)
-                {
+            for (name, inputs) in all_inputs {
+                let mut collector = PhiInputCollector::new();
+                collector.add_snapshot(&inputs);
+                collector.sanitize();
+
+                let 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 continue_merge_bb に生成
-                    LoopSnapshotMergeBox::sanitize_inputs(&mut inputs);
+                    let final_inputs = collector.finalize();
                     let phi_id = ops.f.next_value_id();
                     crate::mir::ssot::cf_common::insert_phi_at_head(
                         ops.f,
                         continue_merge_bb,
                         phi_id,
-                        inputs,
+                        final_inputs,
                     );
                     phi_id
                 };