docs(phase-285): Complete P4 Post-Completion documentation

Phase 285 P4 Post-Completion activities (2025-12-27):

1. Phase 284 P2 Integration Fix (commit 225600b5f)
   - Fixed phase284_p2_return_in_loop_llvm (unreachable instruction)
   - Integration LLVM tests: 3/3 PASS (no FAIL remaining)

2. ret.py Box-First Refactoring (commits 32aa0ddf6, 5a88c4eb2)
   - Phase 1-2: UnreachableReturnHandlerBox + ReturnTypeAdjusterBox
   - Phase 3: StringBoxerBox + ReturnPhiSynthesizerBox
   - Reduced lower_return(): 166→117 lines (-29%)

3. Code Quality Improvements (4 commits)
   - LLVM exit code SSOT (d7c6df367)
   - nyash_kernel FFI split (798c193cb)
   - LLVM detection consolidation (1869396fd)
   - auto_detect.conf clarity (095213c58)

Final status:
- Integration LLVM tests: 3/3 PASS
- Quick profile: 154/154 PASS
- Current Focus: Phase 29y (post self-host, docs-first)

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

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

docs/development/current/main/10-Now.md

@@ -1,8 +1,8 @@
 # Self Current Task — Now (main)
 
-## Current Focus: Phase 29y（post self-host, docs-first）/ Phase 285 follow-ups
+## Current Focus: Phase 29y（post self-host, docs-first）
 
-Phase 285 の “weak + hidden root + LLVM one-pass（検出/設定含む）” は完了。次は post self-host の Phase 29y（MIR lifecycle vocab freeze相談）を docs-first で進める。
+Phase 285 の "weak + hidden root + LLVM one-pass（検出/設定含む）" は完全完了（P4 Post-Completion 含む）。次は post self-host の Phase 29y（MIR lifecycle vocab freeze相談）を docs-first で進める。
 
 **2025-12-26: Phase 285 P2 完了** ✅
 - weak の意味論（`weak <expr>` + `weak_to_strong()` 成功）を integration smoke で固定
@@ -19,6 +19,18 @@ Phase 285 の “weak + hidden root + LLVM one-pass（検出/設定含む）”
 - `KeepAlive { values, drop_after }` を廃止し、`KeepAlive` / `ReleaseStrong` に命令分離（意図を MIR 語彙で明確化）
 - quick smoke 154/154 PASS 維持
 
+**2025-12-27: Phase 285 P4 Post-Completion** ✅
+- **Phase 284 P2 Integration Fix** (commit `225600b5f`)
+  - `phase284_p2_return_in_loop_llvm` FAIL 修正（unreachable instruction 使用）
+  - Integration LLVM tests: 3/3 PASS（FAIL 残りなし）
+- **ret.py Box-First Refactoring** (commits `32aa0ddf6`, `5a88c4eb2`)
+  - Phase 1-2: UnreachableReturnHandlerBox + ReturnTypeAdjusterBox
+  - Phase 3: StringBoxerBox + ReturnPhiSynthesizerBox
+  - lower_return() 削減: 166→117 lines (-29%)
+- **Code Quality Improvements** (commits `d7c6df367`, `798c193cb`, `1869396fd`, `095213c58`)
+  - LLVM exit code SSOT化、nyash_kernel FFI分割、LLVM検出集約、auto_detect.conf明確化
+- quick smoke 154/154 PASS 維持
+
 **2025-12-26: Phase 285 P0 完了** ✅
 - 言語 SSOT との境界明文化（lifecycle.md, types.md）
 - 用語・禁止事項・VM/LLVM差分分類を固定

docs/development/current/main/phases/phase-285/README.md

@@ -90,11 +90,57 @@ MIR 命令を “スコープ維持 / 上書きdrop” に分離して、言語
 
 **目的**: LLVM build でも quick 154/154 PASS（SKIP なし）。
 
-- `apps/tests/phase285_weak_basic.hako` は “upgrade succeeds while strong ref alive” のみに絞る（weak の最小意味論固定）。
+- `apps/tests/phase285_weak_basic.hako` は "upgrade succeeds while strong ref alive" のみに絞る（weak の最小意味論固定）。
 - `tools/smokes/v2/profiles/quick/lifecycle/phase285_weak_basic_llvm.sh` は exit code をゲートにし、stdout 依存を避ける（LLVM harness はログが混入し得る）。
 
 **補足（残課題）**:
-- LLVM harness では “boxed integer handle vs integer literal” の比較が揺れる可能性があるため、weak_basic では field/value 比較を扱わない（別タスク化）。
+- LLVM harness では "boxed integer handle vs integer literal" の比較が揺れる可能性があるため、weak_basic では field/value 比較を扱わない（別タスク化）。
+
+## P4 Post-Completion: Integration LLVM Tests & Code Quality (2025-12-27)
+
+**Phase 284 P2 Integration Fix** (commit `225600b5f`):
+- **Problem**: `phase284_p2_return_in_loop_llvm` failing with LLVM type error
+  - MIR generates unreachable blocks with `{"op": "ret", "value": null}`
+  - LLVM error: `ret void` in `i64 main()` function
+- **Solution**: Use `builder.unreachable()` for Fail-Fast principle
+  - Type-safe: Satisfies LLVM type checker without return value
+  - Fail-Fast: Immediate crash if unreachable block is executed
+  - File: `src/llvm_py/instructions/ret.py` (Lines 47-59)
+- **Verification**:
+  - ✅ phase284_p2_return_in_loop_llvm: PASS (exit 7)
+  - ✅ phase285_p2_weak_upgrade_success_llvm: PASS (exit 2)
+  - ✅ phase285_p2_weak_upgrade_fail_llvm: PASS
+  - ✅ Quick profile: 154/154 PASS (no regression)
+
+**ret.py Box-First Refactoring** (commits `32aa0ddf6`, `5a88c4eb2`):
+- **Phase 1-2**: Extract foundational Boxes
+  - `UnreachableReturnHandlerBox`: Unreachable block handling (Fail-Fast)
+  - `ReturnTypeAdjusterBox`: Type conversion (ptr↔int, width adjustment)
+  - Moved `import os, sys` to file header
+- **Phase 3**: Strategic extraction
+  - `StringBoxerBox`: String pointer (i8*) to handle (i64) conversion
+  - `ReturnPhiSynthesizerBox`: PHI synthesis with zero-like detection
+  - Reduced `lower_return()`: 166→117 lines (-29%)
+  - Total: 205→352 lines (+102 for organization, +4 testable units)
+
+**Code Quality Improvements** (commits `d7c6df367`, `798c193cb`, `1869396fd`, `095213c58`):
+1. **LLVM exit code SSOT** (`d7c6df367`):
+   - Removed unreliable stdout grep fallback
+   - Exit code primary validation (no log pollution)
+2. **nyash_kernel FFI split** (`798c193cb`):
+   - Extracted `ffi/weak.rs` (74 lines)
+   - Reduced lib.rs: 1295→1221 lines (-5.7%)
+3. **LLVM detection consolidation** (`1869396fd`):
+   - Created `can_run_llvm()` function (SSOT)
+   - Updated 9 test files to use unified detection
+4. **auto_detect.conf clarity** (`095213c58`):
+   - `detect_optimal_config(profile)` with parameter
+   - Moved quick SSOT forcing inside function
+
+**Final Status**:
+- Integration LLVM tests: 3/3 PASS (no FAIL remaining)
+- Quick profile: 154/154 PASS
+- Code quality: Box-First principles applied throughout
 
 **LLVM Details**: See [phase-285llvm-1.3-verification-report.md](phase-285llvm-1.3-verification-report.md)
 **Syntax Change**: Phase 285W-Syntax-0 migrates from `weak(x)` function call to `weak x` unary operator