[AZ-266] [AZ-269] [AZ-277] [AZ-280] Cross-cutting log/config + SE3/SHA256 helpers

AZ-266: schema-compliant JSON logging entrypoint, level normalisation,
handler-topology guard, format-error fallback (log_record_schema v1.0.0).
AZ-269: env > YAML > defaults config loader, frozen Config dataclass,
missing-var fail-fast with pointer to .env.example, component-block registry.
AZ-277: GTSAM-backed SE3Utils (matrix<->SE3 + exp/log/adjoint) with strict
orthogonality, dtype, and bottom-row contract enforcement.
AZ-280: atomicwrites-backed write_atomic + independent verify +
order-deterministic aggregate_hash; sidecar format strictness.
pyproject.toml pins gtsam>=4.2,<5.0 and atomicwrites>=1.4,<2.0
(named-backend deps per the AZ-277 / AZ-280 contracts).
139 unit tests pass (44 new). Review verdict: PASS_WITH_WARNINGS;
findings are perf-NFR + journald deferrals, no blocking issues.

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

src/gps_denied_onboard/helpers/__init__.py

@@ -1,5 +1,36 @@
-"""Shared utilities (owned by AZ-264 / E-CC-HELPERS).
+"""Shared utilities (E-CC-HELPERS / AZ-264).
 
-Bootstrap (AZ-263) creates these as importable stubs; concrete implementations are
-filled in by per-helper tasks under AZ-264. See `_docs/02_document/common-helpers/`.
+Each helper has its own task (AZ-276..AZ-283). This package exposes the
+ones that have landed so consumers can depend on a stable public surface
+without reaching into the helper modules directly.
 """
+
+from gps_denied_onboard.helpers.se3_utils import (
+    SE3,
+    Se3InvalidMatrixError,
+    adjoint,
+    exp_map,
+    is_valid_rotation,
+    log_map,
+    matrix_to_se3,
+    se3_to_matrix,
+)
+from gps_denied_onboard.helpers.sha256_sidecar import (
+    SIDECAR_SUFFIX,
+    Sha256Sidecar,
+    Sha256SidecarError,
+)
+
+__all__ = [
+    "SE3",
+    "SIDECAR_SUFFIX",
+    "Se3InvalidMatrixError",
+    "Sha256Sidecar",
+    "Sha256SidecarError",
+    "adjoint",
+    "exp_map",
+    "is_valid_rotation",
+    "log_map",
+    "matrix_to_se3",
+    "se3_to_matrix",
+]