feat(mir): Phase 25.1f完了 - Conservative PHI + ControlForm観測レイヤー

🎉 Conservative PHI Box理論による完全SSA構築

**Phase 7-B: Conservative PHI実装**
- 片方branchのみ定義変数に対応（emit_void使用）
- 全変数にPHI生成（Conservative Box理論）
- Stage-1 resolver全テスト緑化（3/3 PASS）

**Phase 25.1f: ControlForm観測レイヤー**
- LoopShape/IfShape/ControlForm構造定義
- Loop/If統一インターフェース実装
- debug_dump/debug_validate機能追加
- NYASH_CONTROL_FORM_TRACE環境変数対応

**主な変更**:
- src/mir/builder/phi.rs: Conservative PHI実装
- src/mir/control_form.rs: ControlForm構造（NEW）
- src/mir/loop_builder.rs: LoopForm v2デフォルト化

**テスト結果**:
✅ mir_stage1_using_resolver_min_fragment_verifies
✅ mir_stage1_using_resolver_full_collect_entries_verifies
✅ mir_parserbox_parse_program2_harness_parses_minimal_source

🤖 Generated with Claude Code
Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: ChatGPT <chatgpt@openai.com>

src/mir/loop_builder.rs

@@ -6,6 +6,7 @@
  */
 
 use super::{BasicBlockId, ConstValue, MirInstruction, ValueId};
+use crate::mir::control_form::{ControlForm, IfShape, LoopShape, is_control_form_trace_on};
 use crate::mir::phi_core::loop_phi::IncompletePhi;
 use crate::mir::phi_core::loopform_builder::{LoopFormBuilder, LoopFormOps};
 use crate::ast::ASTNode;
@@ -342,7 +343,6 @@ impl<'a> LoopBuilder<'a> {
 
         // Jump to latch if not already terminated
         let actual_latch_id = if !is_current_block_terminated(self.parent_builder)? {
-            let cur_body_end = self.current_block()?;
             self.emit_jump(latch_id)?;
             latch_id
         } else {
@@ -375,6 +375,38 @@ impl<'a> LoopBuilder<'a> {
         // Pop loop context
         crate::mir::builder::loops::pop_loop_context(self.parent_builder);
 
+        // ControlForm 観測: 環境フラグ（未設定時は既定ON）のとき LoopShape をダンプ
+        if is_control_form_trace_on() {
+            // continue / break のターゲットブロックをユニーク化して収集
+            use std::collections::HashSet;
+            let mut cont_set: HashSet<BasicBlockId> = HashSet::new();
+            let mut break_set: HashSet<BasicBlockId> = HashSet::new();
+            for (bb, _) in &self.continue_snapshots {
+                cont_set.insert(*bb);
+            }
+            for (bb, _) in &self.exit_snapshots {
+                break_set.insert(*bb);
+            }
+            let continue_targets: Vec<BasicBlockId> = cont_set.into_iter().collect();
+            let break_targets: Vec<BasicBlockId> = break_set.into_iter().collect();
+
+            let loop_shape = LoopShape {
+                preheader: preheader_id,
+                header: header_id,
+                body: body_id,
+                latch: latch_id,
+                exit: exit_id,
+                continue_targets,
+                break_targets,
+            };
+            let form = ControlForm::from_loop(loop_shape.clone());
+            form.debug_dump();
+            #[cfg(debug_assertions)]
+            if let Some(ref func) = self.parent_builder.current_function {
+                loop_shape.debug_validate(func);
+            }
+        }
+
         // Return void value
         let void_dst = self.new_value();
         self.emit_const(void_dst, ConstValue::Void)?;
@@ -486,7 +518,7 @@ impl<'a> LoopBuilder<'a> {
         }
 
         // Add PHI nodes for new pinned variables in header block
-        for (name, value, preheader_value) in new_pinned_vars {
+        for (name, _value, preheader_value) in new_pinned_vars {
             let phi_id = self.new_value();
             self.emit_phi_at_block_start(header_id, phi_id, vec![(preheader_id, preheader_value)])?;
             // Update variable map to use PHI value
@@ -1156,6 +1188,22 @@ impl<'a> LoopBuilder<'a> {
             &else_var_map_end_opt,
             None,
         )?;
+
+        // ControlForm 観測: 環境フラグ（未設定時は既定ON）のとき IfShape をダンプ
+        if is_control_form_trace_on() {
+            let if_shape = IfShape {
+                cond_block: pre_branch_bb,
+                then_block: then_bb,
+                else_block: Some(else_bb),
+                merge_block: merge_bb,
+            };
+            let form = ControlForm::from_if(if_shape.clone());
+            form.debug_dump();
+            #[cfg(debug_assertions)]
+            if let Some(ref func) = self.parent_builder.current_function {
+                if_shape.debug_validate(func);
+            }
+        }
         let void_id = self.new_value();
         self.emit_const(void_id, ConstValue::Void)?;
         // Pop merge debug region