# Batch 49 — Implementation Report (Cycle 1)

**Tasks**: AZ-526 (Hygiene — consolidate `_iso_ts_from_clock` into `helpers/iso_timestamps.py`)
**Date**: 2026-05-13
**Cycle**: 1
**Status**: COMPLETE (review verdict: PASS_WITH_WARNINGS, one Low / Scope drift finding)

## What was done

Added a second public function `iso_ts_from_clock(clock: Clock) -> str` to the Layer-1 helper module that AZ-508 (Batch 48) introduced. Migrated the four duplicate `_iso_ts_from_clock` definitions to import from the helper, closing cumulative review 46-48 Findings F1 (Medium / Maintainability) and F3 (Low / Maintainability).

### Files added
- _None._ Reused `src/gps_denied_onboard/helpers/iso_timestamps.py` and `tests/unit/test_az508_iso_timestamps.py`.

### Files changed (8)

| File | Change |
|------|--------|
| `src/gps_denied_onboard/helpers/iso_timestamps.py` | Added `iso_ts_from_clock(clock)`; extended module docstring; added `TYPE_CHECKING`-guarded `Clock` import. |
| `src/gps_denied_onboard/helpers/__init__.py` | Re-exported `iso_ts_from_clock`; added to `__all__`. |
| `src/gps_denied_onboard/components/c2_vpr/net_vlad.py` | Deleted local module-level `def _iso_ts_from_clock`; added `from gps_denied_onboard.helpers.iso_timestamps import iso_ts_from_clock as _iso_ts_from_clock`. |
| `src/gps_denied_onboard/components/c2_vpr/ultra_vpr.py` | Same as `net_vlad.py`. |
| `src/gps_denied_onboard/components/c2_vpr/_faiss_bridge.py` | Rewrote `FaissBridge._iso_ts_from_clock` body to `return iso_ts_from_clock(self._clock)`; deleted stale meta-rule comment; added helper import. |
| `src/gps_denied_onboard/components/c12_operator_orchestrator/operator_reloc_service.py` | Same as `_faiss_bridge.py`; also removed now-unused `from datetime import datetime, timezone`. |
| `tests/unit/test_az508_iso_timestamps.py` | Added 7 AC tests (AZ-526 AC-1..AC-4); extended `_helper_uses_stdlib_only` to allow `gps_denied_onboard.clock` (Layer-1) and `typing`; updated `_public_surface_is_minimal` to expect `["iso_ts_from_clock", "iso_ts_now"]`; renamed AC-6 test for clarity; added AST scan for stray module-level `iso_ts_from_clock` defs (distinct from the AZ-508 `iso_ts_now` scan). |
| `_docs/02_document/module-layout.md` | Updated `shared/helpers/iso_timestamps` entry to describe both functions and list the c2_vpr / c12_operator_orchestrator consumers. |
| `_docs/02_tasks/_dependencies_table.md` | Added AZ-526 row (2pt, deps AZ-508 + AZ-398); updated date header and totals. |

## AC coverage

| AC | Verified by | Status |
|----|-------------|--------|
| AC-1 (import + callable) | `test_az526_ac1_import_and_call_returns_str` | PASS |
| AC-2 (Z-suffix nanosecond regex) | `test_az526_ac2_format_matches_canonical_ns_regex`, `test_az526_ac2_fromisoformat_truncated_roundtrip_yields_utc` | PASS |
| AC-3 (nanosecond accuracy) | `test_az526_ac3_nanosecond_fraction_preserved_verbatim`, `test_az526_ac3_sub_second_distinguishability` | PASS |
| AC-4 (call-site migration) | `test_ac4_az526_no_module_level_iso_ts_from_clock_outside_helper`, `test_az526_ac4_all_four_call_sites_import_from_helper`, `test_az526_ac4_no_module_level_local_iso_ts_from_clock_body_in_callers` | PASS (with Spec-Drift caveat — see below) |
| AC-5 (FDR schema tests unmodified) | `tests/unit/test_az272_fdr_record_schema.py` re-run unmodified (33 tests pass) | PASS |
| AC-6 (AZ-270 lint passes) | `tests/unit/test_az270_compose_root.py` (8 tests pass) | PASS |

## Test results

- `tests/unit/test_az508_iso_timestamps.py` — **18 / 18 PASS** (8 pre-existing + 10 added/updated for AZ-526).
- `tests/unit/test_az272_fdr_record_schema.py` — **33 / 33 PASS** unmodified.
- `tests/unit/test_az270_compose_root.py` — **8 / 8 PASS**.
- Component sibling tests for the 4 migrated files (`c2_vpr/`, `c12_operator_orchestrator/`) plus adjacent suites — **186 / 186 PASS** (`pytest -k "az338 or az337 or az489 or az490 or faiss or operator_reloc or net_vlad or ultra_vpr or _faiss"`).
- `ruff check` on all 7 changed source/test files — clean.

## Architectural decisions

1. **`Clock` import via `TYPE_CHECKING`** — `iso_ts_from_clock` only needs `Clock` for its parameter annotation. Importing it under `TYPE_CHECKING` keeps the runtime dependency surface stdlib-only, simplifies the AC-6 lint, and avoids any risk of importing component code at module load.
2. **Output format flip `+00:00` → `Z`** — closes cumulative-46-48 F3. The FDR `ts` field is typed `str` with no schema-level format validation, and the canonical `_TS` fixture in `test_az272_fdr_record_schema.py` already uses `Z`. AC-5 is the safety net; verified by re-running the unmodified schema test suite.
3. **Module-level callers preserved their local symbol via `as _iso_ts_from_clock`** — call-site shape unchanged, so the existing `_emit_fdr` / `_publish` builders read identically. Instance-method callers (FaissBridge, OperatorReLocService) became one-line delegations rather than imports because the call-site is `self._iso_ts_from_clock()` — keeping the method preserves the class's encapsulated public-ish surface in tests that mock it.
4. **Single test file (`test_az508_iso_timestamps.py`)** — the helper module is one cohesive Layer-1 concern; a separate `test_az526_*.py` file would have fragmented the AC walk across two tracker IDs without semantic benefit. The AZ-526 tests are sectioned under a `# AZ-526` divider.

## Spec drift noted (carried into review F1)

AC-4 of the AZ-526 task spec literally reads "zero matches in `c2_vpr/`, `c12_operator_orchestrator/`", but the same spec's § Scope and § Constraints both mandate that the two instance methods "become one-line delegations" — meaning `def _iso_ts_from_clock(self) -> str` remains inside `FaissBridge` and `OperatorReLocService`. Implementation followed Scope/Constraints (delegations preserved). The AST test `test_ac4_az526_no_module_level_iso_ts_from_clock_outside_helper` enforces the **spirit** of AC-4 by scanning only top-level `FunctionDef` nodes; nested class methods are not flagged. Logged as Low / Scope finding F1 in `batch_49_review.md`; no code change.

## Cumulative review obligation

This batch closes cumulative-46-48 F1 and F3. F2 (`_assert_engine_output_dim` near-duplication between `net_vlad.py` and `ultra_vpr.py`) is unchanged — leaving for a future hygiene PBI when a third strategy joins (AZ-339 batch). No new cumulative review window required after this batch (next one is at K=3 → batches 49-51).