[AZ-508] Consolidate _iso_ts_now into helpers/iso_timestamps

Batch 48 / Cycle 1 (greenfield Step 7). Closes cumulative review
batches 31-33 F2 and 28-30 F3 by replacing the duplicated private
_iso_ts_now() one-liners with a single Layer-1 helper:

  src/gps_denied_onboard/helpers/iso_timestamps.py
  iso_ts_now() -> str

Output format matches the canonical FDR _TS fixture
(YYYY-MM-DDTHH:MM:SS.ffffffZ); no FDR schema change.

Migrated call-sites (3): c7_inference/onnx_trt_ep_runtime,
c7_inference/thermal_publisher, plus the 3 c6_tile_cache callers
that previously imported from the local c6_tile_cache/_timestamp
shim (now deleted, superseded by the Layer-1 helper).

Spec drift resolved (Choose A, user-approved): spec listed 5 call
sites + +00:00 regex; on-disk reality at batch start is 3 sites +
Z-suffix matching every existing helper and the FDR _TS fixture.
Spec preamble + AC-2 regex updated in the task file; documented in
batch_48_cycle1_report.md.

Tests: 9 new AC tests (AC-1..AC-7 + Layer-1 invariant +
public-surface defensive); 216 focused tests pass including the
unmodified AZ-272 FDR schema suite and AZ-270 / AZ-507 layering
lints. Verdict: PASS (no findings).

Co-authored-by: Cursor <cursoragent@cursor.com>

_docs/03_implementation/reviews/batch_48_review.md

@@ -0,0 +1,126 @@
+# Code Review Report — Batch 48
+
+**Batch**: 48 (Cycle 1)
+**Tasks**: AZ-508 — ISO-timestamp helper consolidation (2 pt)
+**Date**: 2026-05-13
+**Verdict**: **PASS**
+
+## Findings
+
+_No findings._
+
+## Phase Summary
+
+### Phase 1 — Context Loading
+
+- Task spec: `_docs/02_tasks/todo/AZ-508_hygiene_iso_timestamps_consolidation.md`
+- Cumulative review F2 (batches 31–33) + F3 (batches 28–30) — origin of the
+  hygiene PBI.
+- Changed source files: 1 new helper + 1 helper package re-export +
+  3 c6 import rewires + 2 c7 local-def → import rewires + 1 deleted
+  c6 internal shim.
+- Changed test files: 1 new (`tests/unit/test_az508_iso_timestamps.py`,
+  9 tests covering AC-1..AC-7 + Layer-1 invariant).
+- Changed docs: `module-layout.md` (new `shared/helpers/iso_timestamps`
+  entry) + task spec (drift note + corrected AC-2 regex).
+
+### Phase 2 — Spec Compliance (AC-by-AC)
+
+| AC | Verdict | Evidence |
+|----|---------|----------|
+| AC-1 | PASS | `test_ac1_import_and_call_returns_str` |
+| AC-2 | PASS | `test_ac2_format_matches_canonical_regex` + `test_ac2_fromisoformat_roundtrip_yields_utc_aware_datetime` |
+| AC-3 | PASS | `test_ac3_two_successive_calls_are_non_decreasing` |
+| AC-4 | PASS | `test_ac4_no_other_iso_ts_now_definition_exists_in_src` (AST walk over `src/`) + `test_ac4_c6_and_c7_callers_import_from_helpers` |
+| AC-5 | PASS | `tests/unit/test_az272_fdr_record_schema.py` runs unmodified, 100% pass |
+| AC-6 | PASS | `test_ac6_helper_uses_stdlib_only` (AST whitelist) — `pyproject.toml` unchanged |
+| AC-7 | PASS | `tests/unit/test_az270_compose_root.py::test_ac6_only_compose_root_imports_concrete_strategies` passes |
+
+**Spec drift note**: the task spec originally listed 5 call-sites and a
+`+00:00` regex. On-disk reality at implementation time: 3 call-sites
+(c6 already locally consolidated under `_timestamp.py`; `tensorrt_runtime.py`
+had no local copy) and the canonical FDR `ts` format is `Z`-suffix per
+the `_TS = "2026-05-11T00:00:00.000000Z"` fixture. Spec + AC-2 regex
+updated in this batch; no behavioral drift from the spec's actual intent
+("single Layer-1 helper, byte-identical to existing locals, no FDR
+schema change"). User explicitly approved the path via Choose A.
+
+### Phase 3 — Code Quality
+
+- **SRP**: pure stateless free function; no class wrapper (per Constraint
+  § "no class wrapping").
+- **Error handling**: no exception suppression; stdlib `datetime.now`
+  cannot raise under normal operation.
+- **Naming**: `iso_ts_now` matches the canonical name used by all three
+  pre-existing local helpers — no naming churn in call sites
+  (`from … import iso_ts_now as _iso_ts_now` alias preserved in the c7
+  files).
+- **Complexity**: 1 line. n/a.
+- **DRY**: net deletion — eliminates duplication (the explicit goal).
+- **Test quality**: assertions cover format, parseability, monotonicity,
+  absence of stray definitions, import-path enforcement, layer-1
+  invariant, and stdlib-only constraint.
+- **Dead code**: removed unused `from datetime import datetime, timezone`
+  imports from both c7 files after the local `_iso_ts_now` defs were
+  removed (adjacent-hygiene, permitted by `coderule.mdc`).
+
+### Phase 4 — Security Quick-Scan
+
+No SQL, no `subprocess`, no `eval` / `exec`, no secrets, no untrusted
+input handling. Helper is stdlib-only and side-effect-free. n/a.
+
+### Phase 5 — Performance Scan
+
+`datetime.now(timezone.utc).strftime(...)` is the same call all three
+previous local helpers made; zero runtime delta. No new allocations on
+the hot path.
+
+### Phase 6 — Cross-Task Consistency
+
+Single task in batch — no inter-task interface concerns. Helper aligns
+with the existing 8-helper convention (single-file Layer-1 modules under
+`src/gps_denied_onboard/helpers/`, re-exported from `helpers/__init__.py`).
+
+### Phase 7 — Architecture Compliance
+
+- **Layer direction**: helper sits at Layer 1 (Foundation / shared);
+  consumers in c6 (Layer 2 Infrastructure) and c7 (Layer 2
+  Infrastructure) import strictly downward. ✓
+- **Public API respect**: consumers import via
+  `gps_denied_onboard.helpers.iso_timestamps` (or the `helpers` package
+  facade after the `__init__.py` re-export). Internal modules of c6/c7
+  are not imported across components. ✓
+- **No new cyclic deps**: the helper imports only stdlib; no possible
+  cycle. ✓
+- **Duplicate symbols**: eliminated by design — the AC-4 AST walk
+  asserts zero other `iso_ts_now` / `_iso_ts_now` definitions remain
+  anywhere under `src/`. ✓
+- **Cross-cutting concern home**: moved from per-component locals to
+  the canonical `helpers/` directory. ✓
+- **AZ-507 layering lint**: `test_ac6_only_compose_root_imports_concrete_strategies`
+  passes — helper imports are allowed from components. ✓
+- **AZ-270 lint**: passes. ✓
+
+## Pre-existing Findings (NOT introduced by this batch)
+
+The following ruff findings remain in
+`src/gps_denied_onboard/components/c7_inference/onnx_trt_ep_runtime.py`
+but are pre-existing (verified by re-running ruff against `HEAD` before
+this batch's edits):
+
+- **B905** at line 547 — `zip()` without `strict=`.
+- **RUF022** at lines 88–98 — `__all__` not sorted.
+- **RUF023** at lines 127–134 — `__slots__` not sorted.
+
+Per `coderule.mdc` "Pre-existing lint errors should only be fixed if
+they're in the modified area" — left untouched. Candidate for a future
+c7 hygiene PBI.
+
+## Verdict Justification
+
+- Critical findings: 0
+- High findings:     0
+- Medium findings:   0
+- Low findings:      0
+
+**PASS** — proceed to commit per implement skill Step 11.