refactor(normalization): Deprecate LoopWithPost variant

- Mark LoopWithPost enum variant as deprecated (Phase 142 P0)
- Add deprecation to loop_with_post() constructor function
- Document migration path to LoopOnly
- Keep for backward compatibility until full migration

Phase 142 P0: Statement-level normalization makes LoopWithPost obsolete

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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

@@ -26,8 +26,21 @@ pub enum PlanKind {
 
     /// Phase 132-133: loop(true) + post assignments + return
     ///
+    /// **DEPRECATED in Phase 142 P0**: This variant is no longer created by PlanBox.
+    /// Normalization unit changed from "block suffix" to "statement (loop only)".
+    /// Post-loop assignments are now handled by normal MIR lowering, not normalization.
+    ///
+    /// Migration: Use `LoopOnly` instead. Process subsequent statements separately.
+    ///
+    /// This variant is kept for backward compatibility but will be removed
+    /// once all match arms and tests migrate to statement-level normalization.
+    ///
     /// Pattern: loop + N assignments + return
     /// Example: `loop(true) { x = 1; break }; x = x + 2; return x`
+    #[deprecated(
+        since = "Phase 142 P0",
+        note = "Use LoopOnly instead. Normalization unit is now statement-level."
+    )]
     LoopWithPost {
         /// Number of post-loop assignment statements
         post_assign_count: usize,
@@ -45,6 +58,13 @@ impl NormalizationPlan {
     }
 
     /// Create a Phase 132-133 plan (loop + post assignments + return)
+    ///
+    /// **DEPRECATED in Phase 142 P0**: Use `loop_only()` instead.
+    /// Statement-level normalization makes this obsolete.
+    #[deprecated(
+        since = "Phase 142 P0",
+        note = "Use loop_only() instead. Statement-level normalization makes this obsolete."
+    )]
     pub fn loop_with_post(post_assign_count: usize) -> Self {
         // consumed = 1 (loop) + N (assigns) + 1 (return)
         let consumed = 1 + post_assign_count + 1;