[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shim (now deleted, superseded by the Layer-1 helper).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/helpers/__init__.py

@@ -22,6 +22,7 @@ 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.lightglue_runtime import (
     LightGlueConcurrentAccessError,
     LightGlueRuntime,
@@ -83,6 +84,7 @@ __all__ = [
     "adjoint",
     "exp_map",
     "is_valid_rotation",
+    "iso_ts_now",
     "log_map",
     "make_imu_preintegrator",
     "matrix_to_se3",