[AZ-528] Consolidate c1_vio strategy facade orchestration spine

Replace 3-way byte-equivalent orchestration-spine duplication across
okvis2.py / vins_mono.py / klt_ransac.py with a single c1-internal
helper at components/c1_vio/_facade_spine.py. Closes cumulative
review batches 52-54 Finding F1. No behaviour change — all existing
AZ-332 / AZ-333 / AZ-334 AC tests pass unmodified (114 c1_vio tests
green, 237 with adjacent regression suite).

The helper exposes 5 stateless free functions (now_iso, bias_norm,
se3_from_4x4, frame_ts_ns, frame_image) and a FacadeSpine mixin
class providing _classify_state / _tick_lost / _emit_transition.
Concrete strategies inherit the mixin and set spine-required
instance attributes in __init__. Mirrors the AZ-527 precedent for
c2_vpr-side _assert_engine_output_dim consolidation.

New test file test_az528_facade_spine.py covers AC-1..AC-8 with 19
tests, including an AST regression guard that prevents future
re-introduction of the consolidated free functions in any strategy
module, plus a Risk-1 static check that every strategy's __init__
assigns every spine-required attribute.

Archive AZ-528 task spec to done/, bump autodev state to batch 56.

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

_docs/03_implementation/reviews/batch_55_review.md

@@ -0,0 +1,145 @@
+# Code Review Report
+
+**Batch**: 55 (AZ-528 — c1_vio strategy facade orchestration-spine consolidation)
+**Date**: 2026-05-14
+**Verdict**: PASS
+
+## Scope
+
+Single-task hygiene batch closing Finding F1 from
+`_docs/03_implementation/cumulative_review_batches_52-54_cycle1_report.md`.
+Replaces the 3-way byte-equivalent orchestration-spine duplication
+across `okvis2.py`, `vins_mono.py`, `klt_ransac.py` with a single
+c1-internal helper module `_facade_spine.py`. No behaviour change;
+existing AZ-332 / AZ-333 / AZ-334 AC tests pass unmodified.
+
+### Changed files
+
+- `src/gps_denied_onboard/components/c1_vio/_facade_spine.py` — NEW
+  c1-internal helper. Exports 5 stateless free functions (`now_iso`,
+  `bias_norm`, `se3_from_4x4`, `frame_ts_ns`, `frame_image`) and one
+  mixin class (`FacadeSpine`) providing `_classify_state`,
+  `_tick_lost`, `_emit_transition`. 225 lines including docstring +
+  attribute declarations.
+- `src/gps_denied_onboard/components/c1_vio/okvis2.py` — inherit from
+  `FacadeSpine`; import the 5 free functions; delete the 5 local
+  module-level definitions + 3 instance methods + unused
+  `_BIAS_NORM_FLOOR` constant; set 3 new spine-required attributes in
+  `__init__` (`_feature_threshold`, `_producer_id`, `_strategy_label`).
+- `src/gps_denied_onboard/components/c1_vio/vins_mono.py` — same
+  pattern as `okvis2.py`.
+- `src/gps_denied_onboard/components/c1_vio/klt_ransac.py` — same
+  pattern; reuses `_grayscale` (KLT/RANSAC-specific, not consolidated).
+  Spine threshold attribute reads `_cfg.min_features_for_pose` (the
+  KLT-side equivalent of OKVIS2's `degraded_feature_threshold`).
+- `tests/unit/c1_vio/test_az528_facade_spine.py` — NEW. 19 tests
+  covering AC-1..AC-8 + a Risk-1 mitigation test that statically
+  verifies every concrete strategy's `__init__` sets every
+  spine-required attribute.
+
+## Phase 2 — Spec Compliance
+
+| AC    | Test                                                                                          | Verified |
+|-------|-----------------------------------------------------------------------------------------------|----------|
+| AC-1  | `test_ac1_helper_module_exposes_documented_surface`                                           | ✓        |
+| AC-2  | `test_ac2_now_iso_returns_aware_utc_with_plus_offset`                                         | ✓        |
+| AC-3  | `test_ac3_bias_norm_matches_l2_formula` + `test_ac3_bias_norm_includes_gyro_component`        | ✓        |
+| AC-4  | `test_ac4_se3_from_4x4_builds_identity_pose`                                                  | ✓        |
+| AC-5  | `test_ac5_classify_state_returns_init_during_warmup` + tracking + degraded                    | ✓        |
+| AC-6  | `test_ac6_tick_lost_demotes_tracking_to_degraded_first_call` + escalates-to-lost              | ✓        |
+| AC-7  | `test_ac7_emit_transition_no_record_on_steady_state` + one-record-per-change + idempotent     | ✓        |
+| AC-8  | `test_ac8_no_duplicated_free_functions_remain_in_strategy_module` (parametrised over 3 files) | ✓        |
+| AC-9  | Existing AZ-332 / AZ-333 / AZ-334 AC tests — all unmodified, all pass                         | ✓        |
+| AC-10 | `tests/unit/test_az270_compose_root.py::test_ac6_only_compose_root_imports_concrete_strategies` | ✓      |
+
+Test counts:
+
+- `tests/unit/c1_vio/test_az528_facade_spine.py` — 19 tests, all pass.
+- `tests/unit/c1_vio/` total — 120 collected, 114 pass + 6 tier-2
+  skipped (unchanged from pre-AZ-528 state).
+- Adjacent regression suite (`test_az270_compose_root`, `test_az272_fdr_record_schema`,
+  `test_az273_fdr_client_ringbuf`, `test_ac1_scaffold_layout`,
+  `tests/unit/c1_vio/`) — 237 pass + 6 skipped.
+
+Scope discipline: only the 5 files in the task spec's "Included"
+list were touched; no test files outside the new
+`test_az528_facade_spine.py` were modified (AC-9 hard requirement).
+
+## Phase 3 — Code Quality
+
+- **SRP**: `FacadeSpine` owns one responsibility — the c1_vio
+  strategy state machine + FDR `vio.health` record emission. Free
+  functions are stateless pure utilities. Each concrete strategy
+  retains its geometry pipeline + native-binding driver. Clean split.
+- **Naming**: instance attributes the mixin reads are documented in
+  the class docstring; type annotations on the class declare them for
+  IDE / type-checker consumption (Risk 1 mitigation).
+- **Comments**: deferred-consolidation comments referencing this PBI
+  (`# post-AZ-334 hygiene PBI ...`) were removed from
+  `klt_ransac.py` as required by the task spec.
+- **Dead code**: removed `_BIAS_NORM_FLOOR` from `okvis2.py` (verified
+  via grep; not used anywhere in the c1_vio component or
+  cross-component).
+- **DRY**: the consolidation eliminates ~120 lines of duplicated code
+  across the three strategy modules.
+- **Test quality**: every test follows Arrange / Act / Assert; AC
+  tests use parametrisation where appropriate (AC-8 over the 3
+  strategy modules).
+
+## Phase 4 — Security Quick-Scan
+
+N/A — pure refactor, no new inputs, no I/O, no string interpolation
+into queries, no subprocess invocation. The free functions take only
+typed DTOs (`ImuBias`, `NavCameraFrame`, `np.ndarray`); the mixin
+methods only mutate instance state.
+
+## Phase 5 — Performance
+
+No hot-path regression. The mixin call sites are byte-equivalent to
+the previous in-line definitions (same conditionals, same
+`FdrClient.enqueue` invocation, same string interpolation in
+`bias_norm`). The single new indirection is method dispatch through
+the mixin's MRO — Python class-method resolution is O(1) on a fixed
+MRO and is amortised inside the hot-path's existing call overhead.
+
+## Phase 6 — Cross-Task Consistency
+
+Single-task batch — N/A.
+
+## Phase 7 — Architecture Compliance
+
+- **Layer direction**: `_facade_spine.py` imports only from L1
+  substrate (`_types.nav`, `components.c1_vio.errors`,
+  `fdr_client.records`) + stdlib (`datetime`, `math`) + numpy. No
+  imports from a higher layer; no imports from a sibling component.
+- **Public API respect**: `_facade_spine` is underscore-prefixed and
+  NOT in `c1_vio/__init__.py`'s `__all__`. AZ-270's
+  `test_ac6_only_compose_root_imports_concrete_strategies`
+  regression-gate passes. Mirrors the AZ-527 precedent for
+  `_assert_engine_output_dim` in `c2_vpr`.
+- **Cycles**: no new cyclic dependencies. `_facade_spine` is a leaf
+  inside `c1_vio`; the 3 strategy modules already imported from
+  `c1_vio.errors`, so the new sibling import slots in cleanly.
+- **Duplicate symbols**: AC-8 AST regression guard asserts the 5
+  consolidated free-function names appear only in `_facade_spine.py`.
+- **Cross-cutting concerns**: the spine is a c1-internal concern by
+  task-spec decision (component state machines + per-strategy FDR
+  record producers do not generalise across c2..c5; AZ-507 carved
+  that boundary intentionally). NOT hoisted to `shared/helpers/`.
+
+## Findings
+
+None.
+
+## Conclusion
+
+PASS. The consolidation removes ~120 lines of duplication, preserves
+behaviour across all three strategies (verified by 237 passing tests
+in the focused + adjacent suite), and respects the c1_vio component
+boundary. The cumulative review batches 52-54 Finding F1 is closed.
+
+Risk 4 (mixin couples future divergence) is mitigated by the task
+spec's documented escape hatch: future divergent strategies can
+either override mixin methods or use the free functions in isolation.
+The 3 current strategies' state machines are byte-equivalent
+post-refactor, so the mixin is the right shape today.