phase29ao(p21): tighten pattern1 subset to step-only body

src/mir/builder/control_flow/plan/extractors/pattern1.rs

@@ -5,6 +5,7 @@ use crate::ast::{ASTNode, BinaryOperator};
 
 // Phase 282 P9a: Use common_helpers
 use super::common_helpers::has_control_flow_statement as common_has_control_flow;
+use crate::mir::builder::control_flow::plan::policies::pattern1_subset_policy::is_pattern1_step_only_body;
 
 #[derive(Debug, Clone)]
 pub(crate) struct Pattern1Parts {
@@ -200,6 +201,10 @@ pub(crate) fn extract_pattern1_plan(
         None => return Ok(None),  // No loop increment found
     };
 
+    if !is_pattern1_step_only_body(body, &parts.loop_var) {
+        return Ok(None);
+    }
+
     Ok(Some(DomainPlan::Pattern1SimpleWhile(Pattern1SimpleWhilePlan {
         loop_var: parts.loop_var,
         condition: condition.clone(),

src/mir/builder/control_flow/plan/facts/pattern1_simplewhile_facts.rs

@@ -6,6 +6,7 @@ use crate::mir::builder::control_flow::plan::extractors::common_helpers::{
     extract_loop_increment_plan, has_break_statement, has_continue_statement, has_if_else_statement,
     has_return_statement,
 };
+use crate::mir::builder::control_flow::plan::policies::pattern1_subset_policy::is_pattern1_step_only_body;
 
 #[derive(Debug, Clone)]
 pub(in crate::mir::builder) struct Pattern1SimpleWhileFacts {
@@ -35,6 +36,10 @@ pub(in crate::mir::builder) fn try_extract_pattern1_simplewhile_facts(
         _ => return Ok(None),
     };
 
+    if !is_pattern1_step_only_body(body, &loop_var) {
+        return Ok(None);
+    }
+
     if !is_increment_step_one(&loop_increment, &loop_var) {
         return Ok(None);
     }

src/mir/builder/control_flow/plan/mod.rs

@@ -40,6 +40,8 @@ pub(in crate::mir::builder) mod composer;
 pub(in crate::mir::builder) mod emit;
 // Phase 29ai P6: Extractors moved into plan layer
 pub(in crate::mir::builder) mod extractors;
+// Phase 29ao P21: Pattern1 subset policy (SSOT gate)
+pub(in crate::mir::builder) mod policies;
 // Phase 29ai P5: JoinIR router → single plan extraction entrypoint
 pub(in crate::mir::builder) mod single_planner;
 

src/mir/builder/control_flow/plan/policies/mod.rs

@@ -0,0 +1,3 @@
+//! Phase 29ao P21: Pattern1 subset policy SSOT
+
+pub(in crate::mir::builder) mod pattern1_subset_policy;

src/mir/builder/control_flow/plan/policies/pattern1_subset_policy.rs

@@ -0,0 +1,42 @@
+//! Phase 29ao P21: Pattern1 subset policy (step-only body)
+
+use crate::ast::{ASTNode, BinaryOperator, LiteralValue};
+
+pub(crate) fn is_pattern1_step_only_body(body: &[ASTNode], loop_var: &str) -> bool {
+    if body.len() != 1 {
+        return false;
+    }
+
+    let ASTNode::Assignment { target, value, .. } = &body[0] else {
+        return false;
+    };
+
+    let ASTNode::Variable { name, .. } = target.as_ref() else {
+        return false;
+    };
+    if name != loop_var {
+        return false;
+    }
+
+    let ASTNode::BinaryOp {
+        operator: BinaryOperator::Add,
+        left,
+        right,
+        ..
+    } = value.as_ref()
+    else {
+        return false;
+    };
+
+    if !matches!(left.as_ref(), ASTNode::Variable { name, .. } if name == loop_var) {
+        return false;
+    }
+
+    matches!(
+        right.as_ref(),
+        ASTNode::Literal {
+            value: LiteralValue::Integer(1),
+            ..
+        }
+    )
+}