[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>

_docs/02_tasks/done/AZ-271_config_precedence_tests.md

@@ -0,0 +1,75 @@
+# Config Precedence Unit Tests
+
+**Task**: AZ-271_config_precedence_tests
+**Name**: Config Precedence Tests
+**Description**: A focused unit-test module that verifies the env > YAML > defaults precedence rule for at least 3 keys per layer (per epic AZ-246 AC-3) and the multi-file YAML merge order (later wins). Companion to AZ-269 / AZ-270.
+**Complexity**: 2 points
+**Dependencies**: AZ-269_config_loader, AZ-270_compose_root
+**Component**: shared.config (cross-cutting; epic AZ-246 / E-CC-CONF)
+**Tracker**: AZ-271
+**Epic**: AZ-246 (E-CC-CONF)
+
+### Document Dependencies
+
+- `_docs/02_document/contracts/shared_config/composition_root_protocol.md` — the contract whose precedence invariant this test suite verifies.
+
+## Problem
+
+The composition_root_protocol contract declares a hard precedence rule. Without explicit, tabular tests covering at least 3 keys per layer, regressions in the loader silently flip precedence and cause field bugs that only show up in production deployments where env overrides matter most.
+
+## Outcome
+
+- A pytest module with clearly-named cases covering precedence for ≥3 keys at each layer.
+- A multi-file YAML merge case proving later paths win over earlier paths.
+- Failure messages name the layer (env / YAML / defaults) so on-call engineers can triage fast.
+
+## Scope
+
+### Included
+
+- Precedence cases for ≥3 keys at each of: env > YAML, YAML > defaults, multi-file YAML merge order.
+- One reachability case driving `compose_root` end-to-end with a stub Config to prove the loader and composition functions integrate cleanly.
+- Test fixtures (in-memory YAML strings + env dict, no real files needed for precedence cases; one tmp_path for the multi-file case).
+
+### Excluded
+
+- Performance microbench — owned by AZ-269.
+- Strategy/build-flag mismatch tests — owned by AZ-270.
+- Per-component config block tests — owned by each component epic.
+
+## Acceptance Criteria
+
+**AC-1: env > YAML for at least 3 keys**
+Given env sets `LOG_LEVEL`, `FDR_QUEUE_SIZE`, `MAVLINK_BAUD` and YAML sets all three to different values
+When `load_config(env, [yaml])` runs
+Then all three resolved values match env
+
+**AC-2: YAML > defaults for at least 3 keys**
+Given env is empty and YAML sets `log.level`, `fdr.queue_size`, `mavlink.baud`
+When `load_config(env, [yaml])` runs
+Then all three resolved values match YAML (not the documented defaults)
+
+**AC-3: Defaults apply for at least 3 keys**
+Given env is empty and YAML omits the three keys
+When `load_config(env, [yaml])` runs
+Then all three resolved values match the documented defaults
+
+**AC-4: Multi-file YAML — later wins**
+Given two YAML paths setting the same key to different values
+When `load_config(env, [first, second])` runs
+Then the resolved value matches the second file
+
+**AC-5: Failure messages name the layer**
+Given a precedence assertion fails
+When pytest reports the failure
+Then the assertion message names which layer's value was expected and which was found
+
+## Non-Functional Requirements
+
+**Reliability**
+- Tests are hermetic: no real env vars consulted, no real YAML files outside tmp_path.
+
+## Constraints
+
+- Test file path is fixed at `tests/unit/shared/config/test_precedence.py` (mirrors the `tests/unit/<component>/` convention from module-layout.md Layout Rule 7 — `shared/config` is the component slug).
+- Cases use the SAME 3 keys per layer to make the test matrix comparable across layers.