[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shim (now deleted, superseded by the Layer-1 helper).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>

tests/unit/test_az508_iso_timestamps.py

@@ -0,0 +1,191 @@
+"""AC tests for AZ-508: ISO-timestamp helper consolidation.
+
+Verifies the `iso_timestamps` helper exposed at
+`gps_denied_onboard.helpers.iso_timestamps.iso_ts_now` — the single
+Layer-1 source for FDR record envelope timestamps that replaced the
+duplicated `_iso_ts_now` one-liners in c6_tile_cache and c7_inference.
+
+Output contract (matches the canonical FDR `_TS` fixture in
+`tests/unit/test_az272_fdr_record_schema.py`):
+
+    YYYY-MM-DDTHH:MM:SS.ffffffZ  (UTC, microsecond precision, ``Z`` suffix)
+"""
+
+from __future__ import annotations
+
+import ast
+import re
+import time
+from datetime import datetime, timezone
+from pathlib import Path
+
+import pytest
+
+from gps_denied_onboard.helpers import iso_ts_now
+from gps_denied_onboard.helpers.iso_timestamps import iso_ts_now as iso_ts_now_direct
+
+_TS_REGEX: re.Pattern[str] = re.compile(
+    r"^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{6}Z$"
+)
+
+_REPO_ROOT: Path = Path(__file__).resolve().parents[2]
+_HELPER_PATH: Path = (
+    _REPO_ROOT / "src" / "gps_denied_onboard" / "helpers" / "iso_timestamps.py"
+)
+_C6_DIR: Path = (
+    _REPO_ROOT / "src" / "gps_denied_onboard" / "components" / "c6_tile_cache"
+)
+_C7_DIR: Path = (
+    _REPO_ROOT / "src" / "gps_denied_onboard" / "components" / "c7_inference"
+)
+
+
+def test_ac1_import_and_call_returns_str() -> None:
+    # Act
+    value = iso_ts_now()
+    # Assert
+    assert isinstance(value, str)
+    assert value, "iso_ts_now() returned an empty string"
+    # Both the package-level and module-level imports must resolve to the
+    # same callable so consumers can reach it either way.
+    assert iso_ts_now is iso_ts_now_direct
+
+
+def test_ac2_format_matches_canonical_regex() -> None:
+    # Act
+    value = iso_ts_now()
+    # Assert
+    assert _TS_REGEX.fullmatch(value), (
+        f"{value!r} does not match the canonical FDR ts format "
+        f"YYYY-MM-DDTHH:MM:SS.ffffffZ"
+    )
+
+
+def test_ac2_fromisoformat_roundtrip_yields_utc_aware_datetime() -> None:
+    # Arrange
+    value = iso_ts_now()
+    iso_with_offset = value.replace("Z", "+00:00")
+
+    # Act
+    parsed = datetime.fromisoformat(iso_with_offset)
+
+    # Assert
+    assert parsed.tzinfo is not None
+    assert parsed.utcoffset() == timezone.utc.utcoffset(parsed)
+
+
+def test_ac3_two_successive_calls_are_non_decreasing() -> None:
+    # Arrange / Act
+    a = iso_ts_now()
+    time.sleep(0.000_005)
+    b = iso_ts_now()
+
+    # Assert (lexicographic comparison is correct for the fixed-width format)
+    assert b >= a, f"expected {b!r} >= {a!r}"
+
+
+def test_ac4_no_other_iso_ts_now_definition_exists_in_src() -> None:
+    """AC-4: a `def _iso_ts_now` / `def iso_ts_now` MUST exist only inside
+    `helpers/iso_timestamps.py`. Any other definition under `src/` means a
+    consumer slipped a copy back in.
+    """
+    # Arrange
+    src_root = _REPO_ROOT / "src"
+    offenders: list[tuple[Path, str]] = []
+
+    # Act
+    for path in src_root.rglob("*.py"):
+        if path == _HELPER_PATH:
+            continue
+        try:
+            tree = ast.parse(path.read_text(encoding="utf-8"))
+        except SyntaxError:
+            continue
+        for node in ast.walk(tree):
+            if isinstance(node, ast.FunctionDef) and node.name in {
+                "iso_ts_now",
+                "_iso_ts_now",
+            }:
+                offenders.append((path.relative_to(_REPO_ROOT), node.name))
+
+    # Assert
+    assert offenders == [], (
+        f"Found stray `iso_ts_now` definitions outside the helper: {offenders}"
+    )
+
+
+def test_ac4_c6_and_c7_callers_import_from_helpers() -> None:
+    """The 3 migrated call-sites must import from `helpers.iso_timestamps`
+    (directly or via the `helpers` package facade) so future hygiene
+    cycles can rely on the single source of truth.
+    """
+    # Arrange
+    callers = [
+        _C6_DIR / "cache_budget_enforcer.py",
+        _C6_DIR / "postgres_filesystem_store.py",
+        _C6_DIR / "freshness_gate.py",
+        _C7_DIR / "onnx_trt_ep_runtime.py",
+        _C7_DIR / "thermal_publisher.py",
+    ]
+    expected_token = "gps_denied_onboard.helpers.iso_timestamps"
+
+    # Act / Assert
+    for caller in callers:
+        text = caller.read_text(encoding="utf-8")
+        assert expected_token in text, (
+            f"{caller.relative_to(_REPO_ROOT)} does not import "
+            f"`iso_ts_now` from `{expected_token}`"
+        )
+        assert "def _iso_ts_now" not in text, (
+            f"{caller.relative_to(_REPO_ROOT)} still defines a local "
+            "_iso_ts_now (consolidation incomplete)"
+        )
+
+
+def test_ac6_helper_uses_stdlib_only() -> None:
+    """AC-6: no third-party imports inside the helper module."""
+    # Arrange
+    tree = ast.parse(_HELPER_PATH.read_text(encoding="utf-8"))
+    allowed_stdlib = {"datetime", "__future__"}
+
+    # Act
+    imports: list[str] = []
+    for node in ast.walk(tree):
+        if isinstance(node, ast.Import):
+            imports.extend(alias.name for alias in node.names)
+        elif isinstance(node, ast.ImportFrom):
+            if node.module is not None:
+                imports.append(node.module)
+
+    # Assert
+    for name in imports:
+        top = name.split(".")[0]
+        assert top in allowed_stdlib, (
+            f"helpers/iso_timestamps.py imports `{name}`; only stdlib "
+            f"({sorted(allowed_stdlib)}) is allowed by AC-6"
+        )
+
+
+def test_helper_is_layer_1_no_component_imports() -> None:
+    """Layer-1 discipline: the helper MUST NOT import from any component.
+    (Constraint § Layer-1 discipline in the AZ-508 task spec.)
+    """
+    # Arrange
+    text = _HELPER_PATH.read_text(encoding="utf-8")
+
+    # Assert
+    assert "gps_denied_onboard.components" not in text, (
+        "helpers/iso_timestamps.py imports from a component — Layer-1 "
+        "discipline violated"
+    )
+
+
+@pytest.mark.parametrize("expected_field", ["iso_ts_now"])
+def test_helper_public_surface_is_minimal(expected_field: str) -> None:
+    """Defensive: only ``iso_ts_now`` is re-exported from the module."""
+    # Arrange
+    import gps_denied_onboard.helpers.iso_timestamps as module
+
+    # Assert
+    assert expected_field in module.__all__
+    assert module.__all__ == ["iso_ts_now"]