refactor(normalization): Phase 135 P0 - Extend plan to zero post-loop assigns

Generalize NormalizationPlan suffix detection to accept zero post-loop assignments:

Goal: Improve entry point consistency by allowing `loop + assign* + return` (N >= 0)

Implementation:
- Modified plan_box.rs detection logic (only file changed)
- Removed `post_assign_count >= 1` requirement
- Unified Phase 131 (loop + return) and Phase 132-133 (loop + assign+ + return) paths

Changes:
- src/mir/builder/control_flow/normalization/plan_box.rs:
  - Removed assignment count constraint
  - Unified pattern detection: `loop + assign* + return` (N >= 0)
- apps/tests/phase135_loop_true_break_once_post_empty_return_min.hako (new fixture)
- tools/smokes/v2/profiles/integration/apps/phase135_*.sh (new smoke tests)

Pattern support:
- Phase 131/135: loop + return only (consumed: 2, post_assign_count: 0) ✅
- Phase 132: loop + 1 assign + return (consumed: 3, post_assign_count: 1) ✅
- Phase 133: loop + N assigns + return (consumed: 2+N, post_assign_count: N) ✅

Design principles maintained:
- **Minimal change**: Only plan_box.rs modified (execute_box unchanged)
- **SSOT**: Detection logic centralized in plan_box.rs
- **Box-First**: Responsibility separation preserved (Plan/Execute)

Test results:
- Unit tests (plan_box): 9/9 PASS (2 new tests added)
- Phase 135 VM/LLVM EXE: PASS (exit code 1)
- Phase 131 regression: 2/2 PASS (path now unified)
- Phase 133 regression: 2/2 PASS
- cargo test --lib: PASS

Benefits:
- Unified entry point for all loop + post patterns
- Easier maintenance (single detection logic)
- Future extensibility (easy to add new patterns)
- Clear separation of Phase 131 and Phase 132-135 paths

Default behavior unchanged: Dev-only guard maintained

Related: Phase 135 normalization pattern consistency improvement

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

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

apps/tests/phase135_loop_true_break_once_post_empty_return_min.hako

@@ -0,0 +1,28 @@
+// Phase 135 P0: loop(true) break-once with post 0 assigns + return
+//
+// Purpose: Test loop(true) { <assign>* ; break } + return (0 assignments)
+// Expected output: 1
+//
+// Structure:
+//   x = 0         // pre-loop init
+//   loop(true) {  // condition is Bool literal true
+//       x = 1     // body assignment
+//       break     // break at end
+//   }
+//   return x      // direct return (NO post-loop assignments)
+//
+// Difference from Phase 131:
+//   Phase 131: loop only (no return)
+//   Phase 135: loop + return (0 post assignments)
+
+static box Main {
+    main() {
+        local x
+        x = 0
+        loop(true) {
+            x = 1
+            break
+        }
+        return x
+    }
+}