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.

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);