[AZ-271] [AZ-276] [AZ-278] [AZ-282] Finish cross-cutting helpers + relax opencv pin

E-CC-HELPERS closes with the three remaining Layer-1 helpers and
E-CC-CONF closes with the env > YAML > defaults precedence test
gate. All four tickets ship with frozen public surfaces, hermetic
unit tests, and no upward (components.*) imports.

* AZ-271 — tests/unit/shared/config/test_precedence.py (5 ACs + smoke
  test + helper that names the layer in failure messages).
* AZ-282 — helpers/ransac_filter.py: static RansacFilter +
  RansacResult; cv2.setRNGSeed(0) for byte-equal determinism;
  median residual semantics pinned by contract.
* AZ-276 — helpers/imu_preintegrator.py + make_imu_preintegrator;
  GTSAM PreintegratedCombinedMeasurements; strict-monotonic ts_ns
  guard runs before any state mutation. Adjacent hygiene:
  _types/nav.py ImuSample/ImuWindow now use ts_ns:int and the
  spec-mandated ImuBias dataclass.
* AZ-278 — helpers/lightglue_runtime.py: structural R14 fix.
  LightGlueRuntime + non-blocking concurrent-access guard that
  raises rather than serialising. EngineHandle Protocol in
  _types/manifests.py + KeypointSet/CorrespondenceSet in
  _types/matching.py (Protocol surface adds approved by spec).

Dependency conflict (Finding 1, user-approved): gtsam 4.2 (PyPI) is
numpy-1.x-ABI only; opencv-python>=4.12 needs numpy>=2 at runtime.
Resolution: opencv-python pin relaxed to >=4.11.0.86,<4.12. The
D-CROSS-CVE-1 ratchet at ci/opencv_pin_gate.py is held at 4.11.0
with the original 4.12.0 floor restored once a numpy-2-compatible
gtsam wheel ships. Full replay procedure in
_docs/_process_leftovers/2026-05-11_d_cross_cve_1_opencv_pin_deferred.md.

Tests: 294 passed, 2 skipped (cmake/actionlint env-skips,
pre-existing). 43 new tests added for batch 5. Ruff check + format
clean.

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

tests/unit/shared/config/test_precedence.py

@@ -0,0 +1,235 @@
+"""AZ-271 — Config precedence tests (env > YAML > defaults).
+
+Verifies the precedence rule for ≥3 keys at each layer plus the
+multi-file YAML merge order (later wins) per epic AZ-246 AC-3. Tests
+are hermetic: env is passed in via the loader's ``env`` argument and
+YAML is materialised via ``tmp_path``.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+from gps_denied_onboard.config import (
+    Config,
+    FdrConfig,
+    LogConfig,
+    RuntimeConfig,
+    load_config,
+)
+
+REQUIRED_ENV: dict[str, str] = {
+    "GPS_DENIED_FC_PROFILE": "ardupilot_plane",
+    "GPS_DENIED_TIER": "1",
+    "DB_URL": "postgresql://localhost:5432/test",
+    "CAMERA_CALIBRATION_PATH": "/tmp/cal.yaml",
+    "LOG_LEVEL": "INFO",
+    "LOG_SINK": "console",
+    "INFERENCE_BACKEND": "pytorch_fp16",
+    "FDR_PATH": "/tmp/fdr",
+    "TILE_CACHE_PATH": "/tmp/tiles",
+}
+
+
+def _write_yaml(tmp_path: Path, name: str, content: str) -> Path:
+    path = tmp_path / name
+    path.write_text(content)
+    return path
+
+
+def _layer_msg(layer: str, key: str, expected: object, actual: object) -> str:
+    """Standardised assertion message naming the precedence layer (AC-5)."""
+    return (
+        f"precedence layer {layer!r} for key {key!r}: "
+        f"expected {expected!r} (from {layer}), got {actual!r}"
+    )
+
+
+# ---------------------------------------------------------------------------
+# AC-1: env wins over YAML for ≥3 keys (LOG_LEVEL, FDR_QUEUE_SIZE, GPS_DENIED_TIER).
+
+
+def test_ac1_env_wins_over_yaml_for_three_keys(tmp_path: Path) -> None:
+    # Arrange
+    yaml_path = _write_yaml(
+        tmp_path,
+        "base.yaml",
+        """
+log:
+  level: WARN
+fdr:
+  queue_size: 8192
+runtime:
+  tier: 2
+""",
+    )
+    env = dict(REQUIRED_ENV)
+    env["LOG_LEVEL"] = "ERROR"
+    env["FDR_QUEUE_SIZE"] = "16384"
+    env["GPS_DENIED_TIER"] = "1"
+
+    # Act
+    config = load_config(env=env, paths=(yaml_path,))
+
+    # Assert
+    assert config.log.level == "ERROR", _layer_msg("env", "log.level", "ERROR", config.log.level)
+    assert config.fdr.queue_size == 16384, _layer_msg(
+        "env", "fdr.queue_size", 16384, config.fdr.queue_size
+    )
+    assert config.runtime.tier == 1, _layer_msg("env", "runtime.tier", 1, config.runtime.tier)
+
+
+# ---------------------------------------------------------------------------
+# AC-2: YAML wins over defaults for ≥3 keys.
+
+
+def test_ac2_yaml_wins_over_defaults_for_three_keys(tmp_path: Path) -> None:
+    # Arrange
+    yaml_path = _write_yaml(
+        tmp_path,
+        "base.yaml",
+        """
+log:
+  level: DEBUG
+  sink: journald
+fdr:
+  queue_size: 2048
+""",
+    )
+    env = dict(REQUIRED_ENV)
+    # Remove any env keys that map to the YAML overrides — we want YAML > defaults
+    # without env shadowing.
+    for env_key in ("LOG_LEVEL", "LOG_SINK", "FDR_QUEUE_SIZE"):
+        env.pop(env_key, None)
+    # LOG_LEVEL is in REQUIRED_ENV but the loader's required-env gate would
+    # complain; bypass with ``require_env=False`` to keep the test hermetic.
+
+    # Act
+    config = load_config(env=env, paths=(yaml_path,), require_env=False)
+
+    # Assert
+    log_defaults = LogConfig()
+    fdr_defaults = FdrConfig()
+    assert config.log.level == "DEBUG", _layer_msg("yaml", "log.level", "DEBUG", config.log.level)
+    assert config.log.level != log_defaults.level
+    assert config.log.sink == "journald", _layer_msg(
+        "yaml", "log.sink", "journald", config.log.sink
+    )
+    assert config.log.sink != log_defaults.sink
+    assert config.fdr.queue_size == 2048, _layer_msg(
+        "yaml", "fdr.queue_size", 2048, config.fdr.queue_size
+    )
+    assert config.fdr.queue_size != fdr_defaults.queue_size
+
+
+# ---------------------------------------------------------------------------
+# AC-3: defaults apply for ≥3 keys when env + YAML omit them.
+
+
+def test_ac3_defaults_apply_when_layers_silent() -> None:
+    # Arrange — empty YAML, env_default-only-required-vars.
+    env = dict(REQUIRED_ENV)
+    # Strip three env keys so loader falls to defaults for those three.
+    env.pop("LOG_LEVEL")
+    env.pop("LOG_SINK")
+    env.pop("FDR_QUEUE_SIZE", None)  # FDR_QUEUE_SIZE is not required
+
+    # Act
+    config = load_config(env=env, paths=(), require_env=False)
+
+    # Assert
+    log_defaults = LogConfig()
+    fdr_defaults = FdrConfig()
+    runtime_defaults = RuntimeConfig()
+    assert config.log.level == log_defaults.level, _layer_msg(
+        "default", "log.level", log_defaults.level, config.log.level
+    )
+    assert config.log.sink == log_defaults.sink, _layer_msg(
+        "default", "log.sink", log_defaults.sink, config.log.sink
+    )
+    assert config.fdr.queue_size == fdr_defaults.queue_size, _layer_msg(
+        "default", "fdr.queue_size", fdr_defaults.queue_size, config.fdr.queue_size
+    )
+    # Sanity: runtime defaults also intact for keys with NO env override.
+    assert (
+        config.runtime.inference_backend == runtime_defaults.inference_backend
+        or env.get("INFERENCE_BACKEND") == config.runtime.inference_backend
+    )
+
+
+# ---------------------------------------------------------------------------
+# AC-4: multi-file YAML — later wins.
+
+
+def test_ac4_multi_file_yaml_later_wins(tmp_path: Path) -> None:
+    # Arrange
+    first = _write_yaml(
+        tmp_path,
+        "first.yaml",
+        """
+log:
+  level: WARN
+fdr:
+  queue_size: 1024
+""",
+    )
+    second = _write_yaml(
+        tmp_path,
+        "second.yaml",
+        """
+log:
+  level: ERROR
+fdr:
+  queue_size: 8192
+""",
+    )
+    env = dict(REQUIRED_ENV)
+    env.pop("LOG_LEVEL")  # don't let env shadow the YAML precedence test
+
+    # Act
+    config = load_config(env=env, paths=(first, second), require_env=False)
+
+    # Assert
+    assert config.log.level == "ERROR", _layer_msg(
+        "later-yaml", "log.level", "ERROR", config.log.level
+    )
+    assert config.fdr.queue_size == 8192, _layer_msg(
+        "later-yaml", "fdr.queue_size", 8192, config.fdr.queue_size
+    )
+
+
+# ---------------------------------------------------------------------------
+# AC-5: assertion message names the layer (verified by introspecting helper).
+
+
+def test_ac5_failure_messages_name_the_layer() -> None:
+    # Arrange
+    msg = _layer_msg("env", "log.level", "ERROR", "INFO")
+
+    # Assert — message contains the layer name, the key, and both values.
+    assert "env" in msg
+    assert "log.level" in msg
+    assert "ERROR" in msg
+    assert "INFO" in msg
+
+
+# ---------------------------------------------------------------------------
+# Smoke: load_config + compose_root integrate (regression guard).
+
+
+def test_load_config_returns_frozen_config_dataclass() -> None:
+    # Arrange
+    env = dict(REQUIRED_ENV)
+
+    # Act
+    config = load_config(env=env, paths=())
+
+    # Assert
+    import dataclasses
+
+    assert isinstance(config, Config)
+    with pytest.raises(dataclasses.FrozenInstanceError):
+        # frozen=True → cannot mutate
+        config.log = LogConfig()  # type: ignore[misc]