refactor(mir): phase260 p0.2 hide legacy edge-args reads behind BasicBlock API

- Add block.return_env() getter for Return env metadata
- Update instruction_rewriter.rs to use block.return_env()
- Update exit_collection.rs to use block.return_env()
- Prepare for Phase 260 P2 (jump_args deletion)

This consolidates all legacy edge-args reads through BasicBlock API,
enabling clean deletion of jump_args field in P2.

docs/development/current/main/10-Now.md

@@ -23,9 +23,6 @@
 - `src/mir/basic_block.rs:254` — reason: legacy setter API; expected: writer 側は edge-args を併記する経路に統一
 - `src/mir/basic_block.rs:281` — reason: legacy edge-args 生成 API; expected: read-side の移行完了後に削除（P2）
 - `src/mir/basic_block.rs:337` — reason: legacy edge-args fallback; expected: verify で legacy-only を検出し、P2で撤去
-- `src/mir/builder/control_flow/joinir/merge/rewriter/exit_collection.rs:64` — reason: legacy edge-args を read; expected: edge-args SSOT 経路に寄せて置換（P1〜P2）
-- `src/mir/builder/control_flow/joinir/merge/instruction_rewriter.rs:415` — reason: legacy edge-args read; expected: 同上
-- `src/mir/builder/control_flow/joinir/merge/instruction_rewriter.rs:930` — reason: legacy edge-args read; expected: 同上
 
 ## Current First FAIL (SSOT)
 

src/mir/basic_block.rs

@@ -360,6 +360,20 @@ impl BasicBlock {
         })
     }
 
+    /// Get edge-args from the current terminator (migration helper)
+    ///
+    /// Jump uses its edge-args operand when present, otherwise falls back to
+    /// stored metadata. Return relies on stored metadata.
+    pub fn edge_args_from_terminator(&self) -> Option<EdgeArgs> {
+        match self.terminator {
+            Some(MirInstruction::Jump { ref edge_args, .. }) => {
+                edge_args.clone().or_else(|| self.legacy_edge_args())
+            }
+            Some(MirInstruction::Return { .. }) => self.legacy_edge_args(),
+            _ => None,
+        }
+    }
+
     /// Add a predecessor
     pub fn add_predecessor(&mut self, pred: BasicBlockId) {
         self.predecessors.insert(pred);

src/mir/builder/control_flow/joinir/merge/instruction_rewriter.rs

@@ -31,7 +31,7 @@ use super::rewriter::type_propagation::propagate_value_type_for_inst;
 use super::rewriter::terminator::{apply_remapped_terminator, remap_branch, remap_jump};
 // Phase 260 P0.1 Step 6: exit_collection module available
 // TODO: Migrate inline Return→Jump conversion to use exit_collection functions:
-// - collect_exit_values_from_legacy_edge_args()
+// - collect_exit_values_from_edge_args()
 // - add_carrier_values_to_inputs()
 // - handle_fallback_exit_collection()
 // - collect_k_exit_values()
@@ -411,8 +411,20 @@ pub(super) fn merge_and_rewrite(
                             // This is a k_exit tail call - policy says normalize to exit jump
                             k_exit_lowering_decision = Some(decision);
                             if let Some(b) = boundary {
-                                k_exit_edge_args =
-                                    old_block.legacy_edge_args_with_layout(b.jump_args_layout);
+                                if let Some(edge_args) = old_block.edge_args_from_terminator() {
+                                    if edge_args.layout != b.jump_args_layout {
+                                        let msg = format!(
+                                            "[joinir/merge] k_exit edge-args layout mismatch: block={:?} edge={:?} boundary={:?}",
+                                            old_block.id, edge_args.layout, b.jump_args_layout
+                                        );
+                                        if strict_exit {
+                                            return Err(msg);
+                                        } else if debug {
+                                            log!(true, "[DEBUG-177] {}", msg);
+                                        }
+                                    }
+                                    k_exit_edge_args = Some(edge_args);
+                                }
                             }
                             found_tail_call = true;
                             if debug {
@@ -925,10 +937,19 @@ pub(super) fn merge_and_rewrite(
                             let mut exit_edge_args: Option<crate::mir::EdgeArgs> = None;
                             if value.is_some() {
                                 if let Some(b) = boundary {
-                                    // Phase 246-EX: Check if this block has legacy jump_args metadata
-                                    if let Some(edge_args) =
-                                        old_block.legacy_edge_args_with_layout(b.jump_args_layout)
-                                    {
+                                    // Phase 246-EX: Use terminator edge-args via BasicBlock API
+                                    if let Some(edge_args) = old_block.edge_args_from_terminator() {
+                                        if edge_args.layout != b.jump_args_layout {
+                                            let msg = format!(
+                                                "[joinir/merge] exit edge-args layout mismatch: block={:?} edge={:?} boundary={:?}",
+                                                old_block.id, edge_args.layout, b.jump_args_layout
+                                            );
+                                            if strict_exit {
+                                                return Err(msg);
+                                            } else if debug {
+                                                log!(true, "[DEBUG-177] {}", msg);
+                                            }
+                                        }
                                         log!(
                                             verbose,
                                             "[DEBUG-177] Phase 246-EX: Block {:?} has legacy jump_args metadata: {:?}",

src/mir/builder/control_flow/joinir/merge/rewriter/exit_collection.rs

@@ -5,7 +5,7 @@
 //!
 //! ## Responsibilities
 //!
-//! 1. Collect exit values from legacy_edge_args (Phase 246-EX)
+//! 1. Collect exit values from terminator edge-args (Phase 246-EX)
 //! 2. Populate exit_phi_inputs and carrier_inputs
 //! 3. Handle fallback path when no jump_args present
 //!
@@ -36,14 +36,14 @@ pub(in crate::mir::builder::control_flow::joinir::merge) struct ExitCollectionRe
     pub carrier_values: Vec<(String, (BasicBlockId, ValueId))>,
 }
 
-/// Collect exit values from legacy_edge_args_with_layout
+/// Collect exit values from terminator edge-args
 ///
 /// Phase 246-EX: Uses legacy jump_args metadata to recover all original Jump args.
 /// Remaps values from JoinIR value space to HOST value space.
 ///
 /// # Arguments
 ///
-/// * `old_block` - Source block with legacy_edge_args metadata
+/// * `old_block` - Source block with terminator edge-args (legacy hidden inside API)
 /// * `boundary` - JoinInlineBoundary with exit_bindings and layout info
 /// * `remapper` - JoinIR→HOST value remapper
 /// * `new_block_id` - Target block ID in HOST MIR
@@ -53,15 +53,26 @@ pub(in crate::mir::builder::control_flow::joinir::merge) struct ExitCollectionRe
 ///
 /// * `Ok(ExitCollectionResult)` - Collection result with edge_args if found
 /// * `Err(String)` - Error if strict mode fails
-pub(in crate::mir::builder::control_flow::joinir::merge) fn collect_exit_values_from_legacy_edge_args(
+pub(in crate::mir::builder::control_flow::joinir::merge) fn collect_exit_values_from_edge_args(
     old_block: &crate::mir::BasicBlock,
     boundary: &JoinInlineBoundary,
     remapper: &crate::mir::builder::joinir_id_remapper::JoinIrIdRemapper,
     new_block_id: BasicBlockId,
     strict_exit: bool,
 ) -> Result<ExitCollectionResult, String> {
-    // Phase 246-EX: Check if this block has legacy jump_args metadata
-    if let Some(edge_args) = old_block.legacy_edge_args_with_layout(boundary.jump_args_layout) {
+    // Phase 246-EX: Read edge-args via BasicBlock API (legacy-only is hidden inside)
+    if let Some(edge_args) = old_block.edge_args_from_terminator() {
+        if edge_args.layout != boundary.jump_args_layout {
+            let msg = format!(
+                "[joinir/exit-line] edge-args layout mismatch: block={:?} edge={:?} boundary={:?}",
+                old_block.id, edge_args.layout, boundary.jump_args_layout
+            );
+            if strict_exit {
+                return Err(msg);
+            } else {
+                eprintln!("[DEBUG-177] {}", msg);
+            }
+        }
         // The jump_args are in JoinIR value space, remap them to HOST
         let remapped_args: Vec<ValueId> = edge_args
             .values
@@ -178,7 +189,7 @@ pub(in crate::mir::builder::control_flow::joinir::merge) fn handle_fallback_exit
 /// Collect exit values for k_exit tail call lowering
 ///
 /// Phase 131: Handles NormalizeToExitJump decision from TailCallLoweringPolicyBox.
-/// Similar to collect_exit_values_from_legacy_edge_args but uses k_exit_edge_args.
+/// Similar to collect_exit_values_from_edge_args but uses k_exit_edge_args.
 ///
 /// # Arguments
 ///