refactor(joinir): split Pattern2 orchestrator into smaller steps

src/mir/builder/control_flow/joinir/patterns/policies/p5b_escape_derived_policy.rs

@@ -29,12 +29,26 @@ pub fn classify_p5b_escape_derived(
     body: &[ASTNode],
     loop_var_name: &str,
 ) -> P5bEscapeDerivedDecision {
+    let strict = joinir_dev::strict_enabled();
+    let has_ch_init = find_local_init_expr(body, "ch").is_some();
+    let has_ch_reassign = has_assignment_to_var(body, "ch");
+
     let Some(info) = super::super::ast_feature_extractor::detect_escape_skip_pattern(body) else {
+        if strict && has_ch_init && has_ch_reassign {
+            return P5bEscapeDerivedDecision::Reject(error_tags::freeze(
+                "[phase94/body_local_derived/contract/unhandled_reassign] Body-local reassignment to 'ch' detected but escape shape is not recognized",
+            ));
+        }
         return P5bEscapeDerivedDecision::None;
     };
 
     if info.counter_name != loop_var_name {
         // Not the loop counter we lower as the JoinIR loop var; ignore to avoid misrouting.
+        if strict && has_ch_init && has_ch_reassign {
+            return P5bEscapeDerivedDecision::Reject(error_tags::freeze(
+                "[phase94/body_local_derived/contract/loop_counter_mismatch] Body-local reassignment to 'ch' detected but loop counter does not match Pattern2 loop var",
+            ));
+        }
         return P5bEscapeDerivedDecision::None;
     }
 
@@ -55,6 +69,29 @@ pub fn classify_p5b_escape_derived(
     }
 }
 
+fn has_assignment_to_var(body: &[ASTNode], name: &str) -> bool {
+    fn node_has_assignment(node: &ASTNode, name: &str) -> bool {
+        match node {
+            ASTNode::Assignment { target, .. } => is_var_named(target.as_ref(), name),
+            ASTNode::If {
+                then_body,
+                else_body,
+                ..
+            } => {
+                then_body.iter().any(|n| node_has_assignment(n, name))
+                    || else_body
+                        .as_ref()
+                        .map_or(false, |e| e.iter().any(|n| node_has_assignment(n, name)))
+            }
+            ASTNode::Loop { body, .. } => body.iter().any(|n| node_has_assignment(n, name)),
+            ASTNode::ScopeBox { body, .. } => body.iter().any(|n| node_has_assignment(n, name)),
+            _ => false,
+        }
+    }
+
+    body.iter().any(|n| node_has_assignment(n, name))
+}
+
 fn build_recipe_from_info(
     body: &[ASTNode],
     info: &EscapeSkipPatternInfo,