# Step 7: newton-iteration-log-writer

## 읽어야 할 파일

- `AGENTS.md`
- `docs/ARCHITECTURE.md`
- `docs/nonlinear-static-newton-raphson/io.md`
- `docs/nonlinear-static-newton-raphson/formulation.md`
- `docs/nonlinear-static-newton-raphson/implementation-plan.md`
- Step 0 files: `include/fesa/app/output_paths.h`, `src/fesa/app/output_paths.cpp`, `include/fesa/analysis/analysis.h`
- Step 5 files: `include/fesa/analysis/nonlinear_static_control.h`

## 이전 Step 전제

Step 0 gives app-owned log path and lease lifetime. Step 5 defines convergence-channel data that log records consume.

## 소유 파일

- Create: `include/fesa/analysis/newton_iteration_log.h`
- Create: `src/fesa/analysis/newton_iteration_log.cpp`
- Create: `tests/unit/analysis/newton_iteration_log_test.cpp`
- Modify: `src/fesa/CMakeLists.txt`
- Modify: `tests/CMakeLists.txt`

## 인터페이스

```cpp
namespace fesa {

enum class NewtonIterationStage {
  kEntered,
  kKinematicsEvaluated,
  kInternalAssembled,
  kExternalAssembled,
  kSolvedCorrection,
  kConvergenceEvaluated,
  kAccepted,
  kFailed
};

struct NewtonIterationFailure {
  bool present;
  NewtonIterationStage stage;
  std::string diagnostic_code;
  std::string reason;
};

struct NewtonIterationLogRecord {
  std::string step_name;
  std::uint32_t increment_index;
  double load_factor;
  std::uint32_t iteration;
  NewtonIterationStage last_completed_stage;
  NewtonIterationFailure failure;
  std::string transition;
  std::string outcome;
  std::string diagnostic_code;
  NewtonChannelDecision force_residual;
  NewtonChannelDecision moment_residual;
  NewtonChannelDecision translation_correction;
  NewtonChannelDecision rotation_correction;
  bool combined_passed;
};

class NewtonIterationLogWriter {
 public:
  Status Open(const std::filesystem::path& log_path);
  Status Write(const NewtonIterationLogRecord& record);
  Status Flush();
  Status Close();
};

}  // namespace fesa
```

The serializer, not callers, owns the exact 39 labels and fixed order from `io.md`. For evaluated convergence channels it serializes only `relative` or `zero-scale` branch labels from `NewtonConvergenceBranch`. For unevaluated typed fields it writes `not-evaluated(reason)` using `NewtonIterationFailure::reason`. Callers cannot inject arbitrary fields.

## RED

Add tests first.

- `NSNR-LOG-001`: a complete Newton iteration writes exactly one LF-terminated line.
- `NSNR-LOG-002`: line is UTF-8 without BOM, no header, no trailer.
- `NSNR-LOG-003`: line has exactly 39 TAB-separated `label=value` fields in approved order.
- `NSNR-LOG-004`: finite numeric values round-trip without locale dependence.
- `NSNR-LOG-005`: each convergence channel serializes identity, `numerator_norm`, `denominator_scale`, branch, threshold, `threshold_unit`, and pass from `NewtonChannelDecision`.
- `NSNR-LOG-006`: evaluated empty free-channel decisions serialize `numerator_norm=0`, `denominator_scale=0`, threshold `1.0e-3`, the channel-specific zero-scale `threshold_unit`, and branch `zero-scale`; no other branch label appears.
- `NSNR-LOG-007`: combined pass, transition, outcome, failure stage, and diagnostic code are serialized from explicit record fields.
- `NSNR-LOG-008`: early failure fields after the failed stage use `not-evaluated(reason)`.
- `NSNR-LOG-009`: embedded tab, LF, CR, and backslash are escaped deterministically.
- `NSNR-LOG-010`: write/flush/close failure returns `log-output-failure` and prevents success classification.
- `NSNR-LOG-011`: non-Newton controlled failure path can leave a zero-byte log owned by app lifecycle.

### USER-DECISION-003 option-1 revision addendum

Step 7의 completed status, summary, timestamps와 commits는 보존한다. Executor-selected Step 11에서 `tests/unit/analysis/newton_iteration_log_test.cpp`에 다음 RED assertion을 추가한다.

- `NSNR-LOG-012`: synthetic `NewtonChannelDecision`이 아니라 `EvaluateNewtonConvergence`가 만든 residual decisions를 record에 넣는다. Exact-zero matching external channel은 denominator exact `0`, branch `zero-scale`, threshold `1.0e-3`, force 또는 force-length unit으로 직렬화하고, nonzero matching external channel은 retained denominator, branch `relative`, threshold `1.0e-2`, unit `1`로 직렬화한다. 같은 line이 기존 39 labels/order, TAB separation, LF termination을 그대로 유지해야 하므로 기존 selector에서는 RED가 된다.

Run:

```powershell
cmake --build --preset windows-msvc-ninja-debug
ctest --preset windows-msvc-ninja-debug -R "NewtonIterationLog" --output-on-failure
```

Expected: new tests fail. Unexpected pass stops.

## GREEN

Implement a small log writer with explicit open/write/flush/close state, serializer-owned 39-field order, deterministic escaping, finite numeric formatting, `relative`/`zero-scale` convergence branch mapping, typed `not-evaluated(reason)` substitution, and structured `Status` failures. USER-DECISION-003는 evaluator-produced decision semantics만 바꾸므로 log writer production 또는 `nsnr-log-v1` schema 변경은 예상하지 않는다.

## VERIFY

```powershell
cmake --preset windows-msvc-ninja-debug
cmake --build --preset windows-msvc-ninja-debug
ctest --preset windows-msvc-ninja-debug -R "NewtonIterationLog" --output-on-failure
ctest --preset windows-msvc-ninja-debug --show-only=json-v1
ctest --preset windows-msvc-ninja-debug --output-on-failure
```

Use the ignored `.harness/config.json` preset contract. The Harness process must inherit VsDevCmd x64 and `FESA_GTEST_SOURCE_DIR`, `MKL_DIR`, `TBB_DIR`, and `HDF5_DIR` from its parent launch environment.

## 상태 기록

Update only Step 7 status and one-line payload. Executor owns timestamps, commits, retry, and advancement.

## 금지사항

- Do not duplicate per-iteration log lines to stderr. 이유: approved log is file-only.
- Do not accept arbitrary caller-provided field vectors. 이유: io.md requires an exact fixed schema and order.
- Do not write HDF5 trial history. 이유: log is diagnostic and HDF5 stores accepted frames only.
- Do not continue after log write/flush/close failure. 이유: log failure blocks result commit.
- Do not add, remove, rename, reorder, or re-delimit any `nsnr-log-v1` field. 이유: option 1 changes only residual decision values, not the approved 39-field external log contract.