[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/components/c12_operator_orchestrator/operator_reloc_service.py

@@ -14,7 +14,6 @@ auto-retried. The operator decides when to re-issue.
 from __future__ import annotations
 
 import logging
-from datetime import datetime, timezone
 
 from gps_denied_onboard.clock import Clock
 from gps_denied_onboard.components.c12_operator_orchestrator._types import (
@@ -31,6 +30,7 @@ from gps_denied_onboard.fdr_client.records import (
     CURRENT_SCHEMA_VERSION,
     FdrRecord,
 )
+from gps_denied_onboard.helpers.iso_timestamps import iso_ts_from_clock
 
 __all__ = ["OperatorReLocService"]
 
@@ -165,10 +165,7 @@ class OperatorReLocService:
             )
 
     def _iso_ts_from_clock(self) -> str:
-        ns = int(self._clock.time_ns())
-        seconds, fraction_ns = divmod(ns, 1_000_000_000)
-        dt = datetime.fromtimestamp(seconds, tz=timezone.utc)
-        return f"{dt.strftime('%Y-%m-%dT%H:%M:%S')}.{fraction_ns:09d}+00:00"
+        return iso_ts_from_clock(self._clock)
 
 
 def _hint_to_payload(hint: ReLocHint) -> dict[str, object]:

src/gps_denied_onboard/components/c2_vpr/_faiss_bridge.py

@@ -48,6 +48,7 @@ from gps_denied_onboard.fdr_client.records import (
     CURRENT_SCHEMA_VERSION,
     FdrRecord,
 )
+from gps_denied_onboard.helpers.iso_timestamps import iso_ts_from_clock
 
 if TYPE_CHECKING:
     pass
@@ -304,14 +305,4 @@ class FaissBridge:
             )
 
     def _iso_ts_from_clock(self) -> str:
-        # Inlined here rather than importing a shared helper because every
-        # other component that emits FDR records (c12, c11, c5) defines
-        # the same one-liner locally — see meta-rule "Critical Thinking":
-        # the duplication is intentional and trivial, factoring it would
-        # add an L1 import every component already avoids.
-        from datetime import datetime, timezone
-
-        ns = int(self._clock.time_ns())
-        seconds, fraction_ns = divmod(ns, 1_000_000_000)
-        dt = datetime.fromtimestamp(seconds, tz=timezone.utc)
-        return f"{dt.strftime('%Y-%m-%dT%H:%M:%S')}.{fraction_ns:09d}+00:00"
+        return iso_ts_from_clock(self._clock)

src/gps_denied_onboard/components/c2_vpr/net_vlad.py

@@ -105,6 +105,9 @@ from gps_denied_onboard.fdr_client.records import (
     FdrRecord,
 )
 from gps_denied_onboard.helpers.descriptor_normaliser import DescriptorNormaliser
+from gps_denied_onboard.helpers.iso_timestamps import (
+    iso_ts_from_clock as _iso_ts_from_clock,
+)
 
 if TYPE_CHECKING:
     from gps_denied_onboard._types.calibration import CameraCalibration
@@ -372,17 +375,6 @@ class NetVladStrategy:
         )
 
 
-def _iso_ts_from_clock(clock: Clock) -> str:
-    # Same shape every component uses for FDR timestamps; AZ-508 will
-    # consolidate the duplicate helpers across c2/c11/c12/c6.
-    from datetime import datetime, timezone
-
-    ns = int(clock.time_ns())
-    seconds, fraction_ns = divmod(ns, 1_000_000_000)
-    dt = datetime.fromtimestamp(seconds, tz=timezone.utc)
-    return f"{dt.strftime('%Y-%m-%dT%H:%M:%S')}.{fraction_ns:09d}+00:00"
-
-
 def _build_pytorch_build_config(weights_path: Path) -> BuildConfig:
     del weights_path
     return BuildConfig(

src/gps_denied_onboard/components/c2_vpr/ultra_vpr.py

@@ -84,6 +84,9 @@ from gps_denied_onboard.fdr_client.records import (
     FdrRecord,
 )
 from gps_denied_onboard.helpers.descriptor_normaliser import DescriptorNormaliser
+from gps_denied_onboard.helpers.iso_timestamps import (
+    iso_ts_from_clock as _iso_ts_from_clock,
+)
 
 if TYPE_CHECKING:
     from gps_denied_onboard._types.calibration import CameraCalibration
@@ -323,17 +326,6 @@ class UltraVprStrategy:
         )
 
 
-def _iso_ts_from_clock(clock: Clock) -> str:
-    # Same shape every component uses for FDR timestamps; AZ-508 will
-    # consolidate the duplicate helpers across c2/c11/c12/c6.
-    from datetime import datetime, timezone
-
-    ns = int(clock.time_ns())
-    seconds, fraction_ns = divmod(ns, 1_000_000_000)
-    dt = datetime.fromtimestamp(seconds, tz=timezone.utc)
-    return f"{dt.strftime('%Y-%m-%dT%H:%M:%S')}.{fraction_ns:09d}+00:00"
-
-
 def _build_trt_build_config() -> BuildConfig:
     return BuildConfig(
         precision=PrecisionMode.FP16,