[AZ-526] Consolidate _iso_ts_from_clock into helpers/iso_timestamps

Closes cumulative review 46-48 F1 (Medium) + F3 (Low). Adds
iso_ts_from_clock(clock) alongside iso_ts_now() in the Layer-1
helper; migrates four duplicate definitions in c2_vpr (net_vlad,
ultra_vpr, _faiss_bridge) and c12_operator_orchestrator
(operator_reloc_service). Output format flipped +00:00 -> Z to
align with iso_ts_now() and the canonical FDR _TS fixture (FDR
schema test passes unmodified).

18 helper AC tests + 186 sibling tests pass; ruff clean.

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

src/gps_denied_onboard/helpers/__init__.py

@@ -22,7 +22,10 @@ from gps_denied_onboard.helpers.imu_preintegrator import (
     ImuPreintegrator,
     make_imu_preintegrator,
 )
-from gps_denied_onboard.helpers.iso_timestamps import iso_ts_now
+from gps_denied_onboard.helpers.iso_timestamps import (
+    iso_ts_from_clock,
+    iso_ts_now,
+)
 from gps_denied_onboard.helpers.lightglue_runtime import (
     LightGlueConcurrentAccessError,
     LightGlueRuntime,
@@ -84,6 +87,7 @@ __all__ = [
     "adjoint",
     "exp_map",
     "is_valid_rotation",
+    "iso_ts_from_clock",
     "iso_ts_now",
     "log_map",
     "make_imu_preintegrator",