[AZ-386] C5 ESKF baseline: 16-state error-state KF (NumPy)

Implements the mandatory simple-baseline StateEstimator per AC-2.1a
engine-rule at C5 (IT-12 comparative study vs iSAM2). NumPy-only;
no GTSAM dependency so BUILD_STATE_ESKF=ON binaries ship without
GTSAM at all.

- 16-state error vector (pos 3 + vel 3 + rot 3 + ba 3 + bg 3 + dt 1)
  over a textbook nominal-state / error-state ESKF split.
- add_fc_imu: full nonlinear IMU integration + linearised F P F^T + Q
  covariance propagation per IMU sample.
- add_vio: simplified relative-pose update (snapshot-based; baseline
  scope, documented).
- add_pose_anchor: absolute-pose update; integrates BOTH marginals and
  jacobian modes (no skip — ESKF has no graph; AC-4).
- AC-9 divergence test: Mahalanobis r^T S^-1 r > 100 (10 sigma) on the
  innovation covariance S = H P H^T + R.
- AC-5 SPD: Cholesky-positive enforcement on every emitted covariance;
  non-SPD raises EstimatorFatalError and locks state to LOST.
- AC-6 honesty: smoothed_history entries carry smoothed=False; deviation
  from C5 contract Invariant 7 documented in module + report.
- AC-7 / AC-10 BUILD_STATE_ESKF gating: works through existing factory
  infra (state_factory._STATE_BUILD_FLAGS).
- AC-8: SourceLabelStateMachine + FallbackWatcher auto-wired eagerly
  in __init__, same pattern as the iSAM2 estimator.

Tests: 20 new unit tests covering AC-1..AC-10 + robustness checks.
Full suite: 660 passed, 2 skipped (CI-only).

The AZ-386 Jira transition to Done is deferred (Atlassian MCP returned
'Not connected'); recorded in _docs/_process_leftovers/ for replay on
the next autodev invocation per the Leftovers Mechanism.

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

_docs/_autodev_state.md

@@ -8,7 +8,7 @@ status: in_progress
 sub_step:
   phase: 6
   name: implement-tasks
-  detail: "batch 18 of N committed (AZ-387 c5 smoothed-history \u2192 FDR side-channel: per-keyframe FDR emission + timestamp watermark idempotency + AC-4.5 onboard-only path + AC-3/AC-4 cross-task invariants documented)"
+  detail: "batch 19 of N committed (AZ-386 c5 ESKF mandatory simple-baseline: 16-state error-state KF NumPy impl + add_vio relative + add_pose_anchor absolute + Mahalanobis divergence gate AC-9 + source-label SM + AC-5.2 fallback auto-wired + smoothed=False history honesty (Invariant 7 deviation documented) + BUILD_STATE_ESKF flag wiring)"
 retry_count: 0
 cycle: 1
 tracker: jira