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shim (now deleted, superseded by the Layer-1 helper).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

src/gps_denied_onboard/components/c7_inference/onnx_trt_ep_runtime.py

@@ -46,7 +46,6 @@ from __future__ import annotations
 import hashlib
 import os
 from collections.abc import Callable
-from datetime import datetime, timezone
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, Final, Literal
 
@@ -79,6 +78,7 @@ from gps_denied_onboard.fdr_client.records import (
     CURRENT_SCHEMA_VERSION,
     FdrRecord,
 )
+from gps_denied_onboard.helpers.iso_timestamps import iso_ts_now as _iso_ts_now
 from gps_denied_onboard.logging import get_logger
 
 if TYPE_CHECKING:
@@ -166,18 +166,6 @@ def _sha256_of_file(path: Path) -> str:
     return digest.hexdigest()
 
 
-def _iso_ts_now() -> str:
-    """RFC 3339 UTC timestamp with microsecond precision and a ``Z`` suffix.
-
-    Mirrors :func:`components.c6_tile_cache._timestamp.iso_ts_now` —
-    consolidation into ``helpers.iso_timestamp`` is intentionally
-    deferred to the next cross-component hygiene pass (peer imports
-    between c6 and c7 would violate layer-2 horizontal-import etiquette
-    documented in ``module-layout.md``).
-    """
-    return datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%S.%fZ")
-
-
 # ----------------------------------------------------------------------
 # Runtime.
 

src/gps_denied_onboard/components/c7_inference/thermal_publisher.py

@@ -53,7 +53,6 @@ from __future__ import annotations
 
 import threading
 from dataclasses import dataclass
-from datetime import datetime, timezone
 from typing import TYPE_CHECKING, Protocol, runtime_checkable
 
 from gps_denied_onboard._types.thermal import ThermalState
@@ -65,6 +64,7 @@ from gps_denied_onboard.fdr_client.records import (
     CURRENT_SCHEMA_VERSION,
     FdrRecord,
 )
+from gps_denied_onboard.helpers.iso_timestamps import iso_ts_now as _iso_ts_now
 from gps_denied_onboard.logging import get_logger
 
 if TYPE_CHECKING:
@@ -340,11 +340,6 @@ def _default_safe_snapshot(measured_at_ns: int) -> ThermalState:
     )
 
 
-def _iso_ts_now() -> str:
-    """RFC 3339 UTC timestamp with microsecond precision and ``Z`` suffix."""
-    return datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%S.%fZ")
-
-
 class _JtopSource:
     """``jtop`` (jetson-stats) thermal source — Tier-2 production path.