[AZ-333] C1 VINS-Mono strategy — research-only comparative VIO

VinsMonoStrategy: Python facade conforming to AZ-331 Protocol; mirrors
the AZ-332 OKVIS2 facade so the AZ-331 factory + IT-12 comparative
harness can treat both as drop-in substitutable. Native binding is a
pybind11 skeleton compiled behind BUILD_VINS_MONO=ON (default OFF for
airborne / operator-tooling / replay-cli per module-layout.md
Build-Time Exclusion Map). Real vins_estimator wiring is the Tier-2
follow-up.

VinsMonoConfig added to c1_vio/config.py with sliding-window /
feature-tracker / marginalisation / opt-iteration knobs plus
__post_init__ validation; exported through the package __init__.

cpp/vins_mono/CMakeLists.txt replaces the AZ-263 placeholder with full
pybind11 wiring: Risk-1 mitigation forces VINS_MONO_USE_ROS=OFF;
Risk-2 mitigation links Eigen from the same cpp/_third_party/eigen pin
as OKVIS2; Risk-3 mitigation enforces BUILD_VINS_MONO=OFF in
deployment binaries via the gate at the top of the file.

Tests: 17 new in test_vins_mono_strategy.py (15 pass + 2 tier2 skip);
fake_vins_mono_binding fixture added to conftest.py mirroring the
fake_okvis2_binding pattern; test_protocol_conformance updated to drop
vins_mono from _STRATEGIES_WITHOUT_PY_MODULE so the existing
parametrised factory tests route through the new strategy.

Focused c1_vio suite: 72 passed, 4 skipped. Full suite: 1788 passed,
1 unrelated pre-existing flake (c12 cold-start perf, env-bound).

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

_docs/03_implementation/batch_53_cycle1_report.md

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+# Batch 53 — Cycle 1 Report
+
+**Date**: 2026-05-14
+**Tasks**: AZ-333 (C1 VINS-Mono Strategy)
+**Verdict**: COMPLETE — PASS_WITH_WARNINGS
+
+## Summary
+
+Implemented `VinsMonoStrategy`, the research-only loosely-coupled
+comparative VIO that participates in the IT-12 comparative-study
+research binary only. Mirrors the AZ-332 OKVIS2 facade pattern
+deliberately so the AZ-331 factory can treat both strategies as
+drop-in substitutable. Native binding is a pybind11 skeleton compiled
+behind `BUILD_VINS_MONO=ON` (default OFF for airborne /
+operator-tooling / replay-cli); estimator wiring is the Tier-2
+follow-up.
+
+## Files added / modified
+
+### Added (4)
+
+- `src/gps_denied_onboard/components/c1_vio/vins_mono.py` — Python
+  facade, 533 lines.
+- `src/gps_denied_onboard/components/c1_vio/_native/vins_mono_binding.cpp`
+  — pybind11 binding skeleton, ~275 lines.
+- `tests/unit/c1_vio/test_vins_mono_strategy.py` — AC-1..AC-10 +
+  tier2 perf/honesty tests, 518 lines, 17 tests (15 pass, 2 skip).
+- `_docs/03_implementation/reviews/batch_53_review.md` — code review
+  report.
+
+### Modified (4)
+
+- `src/gps_denied_onboard/components/c1_vio/__init__.py` — export
+  `VinsMonoConfig`.
+- `src/gps_denied_onboard/components/c1_vio/config.py` — add
+  `VinsMonoConfig` dataclass + `vins_mono` field on `C1VioConfig`;
+  `__all__` updated; `KNOWN_STRATEGIES` already had `vins_mono`.
+- `cpp/vins_mono/CMakeLists.txt` — replaced AZ-263 placeholder with
+  full pybind11 + linker wiring; gated by `BUILD_VINS_MONO`; ROS-strip
+  flag forced OFF (Risk-1); links Eigen from
+  `cpp/_third_party/eigen/` shared with OKVIS2 (Risk-2).
+- `tests/unit/c1_vio/conftest.py` — extend with `FakeVinsMonoBackend`
+  + 3 fake exception types + `fake_vins_mono_binding` fixture.
+- `tests/unit/c1_vio/test_protocol_conformance.py` — drop `vins_mono`
+  from `_STRATEGIES_WITHOUT_PY_MODULE` so the existing parametrised
+  factory test routes through the new strategy correctly (the
+  "module missing" branch is now strictly `klt_ransac`-only until
+  AZ-334 lands).
+
+## AC coverage (AC-1..AC-10 + NFR-perf-document)
+
+All 10 ACs mapped to passing tests (see `batch_53_review.md` Phase 2
+table). AC-9 + NFR-perf are tier2-tagged per the carry-over plan;
+they skip on macOS dev + GitHub Actions Linux runner with
+`Tier-2-only test; set GPS_DENIED_TIER=2 to run`.
+
+## Test results
+
+### Focused suite — `tests/unit/c1_vio/`
+
+```
+72 passed, 4 skipped in 1.14s
+```
+
+The 4 skips are the 2 OKVIS2 tier2 tests + the 2 new VINS-Mono tier2
+tests (`test_ac9_*` and `test_nfr_perf_*`).
+
+### Adjacent regression — config / compose-root / inference shim
+
+```
+22 passed in 2.87s
+```
+
+(`test_az269_config_loader.py`, `test_az270_compose_root.py`,
+`test_az507_inference_errors_shim.py` — all green; the new
+`VinsMonoConfig` registration did not break the schema-loader or
+compose-root layered-import guards.)
+
+### Full suite
+
+```
+1 failed, 1788 passed, 82 skipped in 79.15s
+```
+
+The single failure is a pre-existing environment-dependent perf flake:
+
+- `tests/unit/c12_operator_orchestrator/test_cli_console_script.py::test_cold_start_under_500ms_p99`
+  — measures CLI cold-start wall-clock; reports `worst-after-trim
+  997.4 ms` vs the `≤ 500 ms` NFR target. macOS dev runner cannot
+  hit this on a cold spawn; the test is environment-bound to Linux
+  CI hardware. No touchpoint to c1_vio. Reported to the user.
+
+## Architectural decisions
+
+- **Mirroring OKVIS2 1:1**: the constructor / state machine / except
+  ladder in `vins_mono.py` is a deliberate copy of `okvis2.py`. The
+  AZ-331 factory + IT-12 harness require shape-compatibility; doing
+  the consolidation now (before AZ-334's KltRansac shape is visible)
+  would over-fit. Tracked as Low-severity finding F1 in the review;
+  scheduled for the post-AZ-334 hygiene PBI (precedent: AZ-340 →
+  AZ-527 for c2_vpr).
+- **Test fake mirroring**: same logic for `FakeVinsMonoBackend` vs
+  `FakeOkvis2Backend`. The shared `ScriptedOutput` dataclass and
+  `_make_default_payload` helper ARE reused (the productive cut);
+  the backend class duplication is the deferred-consolidation slice.
+- **CMakeLists ROS-strip + Eigen pin**: explicit Risk-1 mitigation
+  (`VINS_MONO_USE_ROS=OFF` forced), Risk-2 mitigation (Eigen pin
+  from the shared `cpp/_third_party/eigen/`), Risk-3 mitigation (the
+  binding never builds when `BUILD_VINS_MONO=OFF`, so deployment
+  binaries stay clean).
+- **Skeleton binding throws on first frame**: matches OKVIS2 — a
+  research binary that loads the `.so` before tier-2 wires the real
+  `vins_estimator::Estimator` cannot silently emit misleading poses
+  (`VinsMonoFatalException` from `_drive_estimator`).
+- **NFR-perf is recorded, not bounded**: per task spec, VINS-Mono is
+  exempt from C1-PT-01's 80 ms p95. The tier2 perf test asserts only
+  that `process_frame` completes 200× without deadlock and that p95
+  is below a loose 5-second sanity ceiling. The Step 9 / E-BBT
+  comparative-study report consumes the actual p50/p95 number.
+
+## Out of scope / deferred
+
+- Real `vins_estimator::Estimator` wiring inside the binding — Tier-2
+  follow-up; not required for the AZ-333 facade ACs.
+- KltRansac strategy (AZ-334) — separate batch.
+- Warm-start hint persistence (AZ-335) — separate batch, depends on
+  AZ-333 + AZ-334.
+- Strategy-facade consolidation hygiene PBI — informally tracked,
+  formal PBI to be raised by the next cumulative review (batches
+  52-54) once all three c1_vio strategies are landed.