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>

src/mir/builder/control_flow/normalization/README.md

@@ -87,13 +87,13 @@ pub enum PlanKind {
 
 ---
 
-## Pattern Detection (Phase 131-133)
+## Pattern Detection (Phase 131-135)
 
 ### Phase 131: Loop-Only
-- Pattern: `loop(true) { ... break }`
+- Pattern: `loop(true) { ... break }` (single statement, no return)
 - Consumed: 1 statement
 - Kind: `PlanKind::LoopOnly`
-- Example: `loop(true) { x = 1; break }; return x`
+- Example: `loop(true) { x = 1; break }`
 
 ### Phase 132: Loop + Single Post
 - Pattern: `loop(true) { ... break }; <assign>; return <expr>`
@@ -107,6 +107,14 @@ pub enum PlanKind {
 - Kind: `PlanKind::LoopWithPost { post_assign_count: N }`
 - Example: `loop(true) { x = 1; break }; x = x + 2; x = x + 3; return x`
 
+### Phase 135: Loop + Return (Zero Post Assigns) **NEW**
+- Pattern: `loop(true) { ... break }; return <expr>` (0 post-loop assignments)
+- Consumed: 2 statements (loop, return)
+- Kind: `PlanKind::LoopWithPost { post_assign_count: 0 }`
+- Example: `loop(true) { x = 1; break }; return x`
+- **Improvement**: Unifies Phase 131 and Phase 132-133 patterns under `LoopWithPost` enum
+- **Compatibility**: Phase 131 (loop-only, no return) remains as `PlanKind::LoopOnly`
+
 ---
 
 ## Contract
@@ -127,7 +135,7 @@ pub enum PlanKind {
 **Invariants**:
 - `consumed <= remaining.len()` (never consume more than available)
 - If `requires_return` is true, `remaining[consumed-1]` must be Return
-- Post-assign patterns require `consumed >= 3` (loop + assign + return minimum)
+- Post-assign patterns require `consumed >= 2` (loop + return minimum, Phase 135+)
 
 ### NormalizationExecuteBox::execute()
 
@@ -172,10 +180,13 @@ pub enum PlanKind {
 - Phase 131 pattern detection (loop-only)
 - Phase 132 pattern detection (loop + single post)
 - Phase 133 pattern detection (loop + multiple post)
+- **Phase 135 pattern detection (loop + zero post)** **NEW**
 - Return boundary detection (consumed stops at return)
+- **Return boundary with trailing statements** **NEW**
 - Non-matching patterns (returns None)
 
 ### Regression Smokes
+- **Phase 135**: `phase135_loop_true_break_once_post_empty_return_{vm,llvm_exe}.sh` **NEW**
 - Phase 133: `phase133_loop_true_break_once_post_multi_add_{vm,llvm_exe}.sh`
 - Phase 132: `phase132_loop_true_break_once_post_add_llvm_exe.sh`
 - Phase 131: `phase131_loop_true_break_once_{vm,llvm_exe}.sh`