[AZ-776] Open-loop ESKF composition profile via c4_pose.enabled

ADR-012: add c4_pose.enabled (default True) and enforce the
(c4_pose.enabled, c5_state.strategy) 2x2 pairing matrix at compose
time. When enabled=false, compose_root removes c4_pose from the
selection map and build_pre_constructed omits c5_isam2_graph_handle.
Replay protocol Invariant 13 owns the gate. Tier-2 conftest YAML
writes the open-loop profile; un-xfails AC-1/2/5 and both AC-6
variants in Derkachi (AC-3 stays xfailed for AZ-777). 319/319
runtime_root + c4_pose + c5_state tests green.

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

src/gps_denied_onboard/runtime_root/airborne_bootstrap.py

@@ -1135,6 +1135,33 @@ def _build_c3_feature_extractor(config: Config) -> FeatureExtractor:
     return OpenCvOrbExtractor()
 
 
+def _c4_pose_disabled(config: Config) -> bool:
+    """True iff the airborne open-loop ESKF composition profile is active.
+
+    AZ-776 / ADR-012: when ``config.components["c4_pose"].enabled`` is
+    explicitly ``False`` the composition root strips ``c4_pose`` from
+    the component graph and the C4 wrapper never runs. The eager
+    ``(estimator, handle)`` build inside :func:`build_pre_constructed`
+    therefore has no consumer for the iSAM2 graph handle slot
+    (``c5_isam2_graph_handle``); leaving the slot absent makes the
+    "no C4" property visible at the pre-bootstrap layer too.
+
+    Returns ``False`` when the block is absent or when ``enabled`` is
+    missing / true — the full GTSAM profile (ADR-003 steady state)
+    remains the default airborne pipeline. Replay-mode component-block
+    omission is handled separately by
+    :func:`_replay_omits_component_block`.
+    """
+    components = getattr(config, "components", None) or {}
+    if not isinstance(components, Mapping):
+        return False
+    block = components.get("c4_pose")
+    if block is None:
+        return False
+    enabled = getattr(block, "enabled", True)
+    return not bool(enabled)
+
+
 def _replay_omits_component_block(config: Config, block_name: str) -> bool:
     """True iff replay-mode :class:`Config` has no ``components[block_name]`` entry.
 
@@ -1301,7 +1328,14 @@ def build_pre_constructed(config: Config) -> dict[str, Any]:
             fdr_client=constructed["c13_fdr"],
             tile_store=constructed.get("c6_tile_store"),
         )
-        constructed["c5_isam2_graph_handle"] = handle
+        # AZ-776 / ADR-012 open-loop ESKF profile: c4_pose.enabled=False
+        # strips c4_pose from the composition graph, so the
+        # c5_isam2_graph_handle slot has no consumer. ESKF returns
+        # handle=None by design; omitting the slot makes the open-loop
+        # property visible at the pre_constructed layer and refuses the
+        # invariant "handle is None means C4 will crash on read".
+        if not _c4_pose_disabled(config):
+            constructed["c5_isam2_graph_handle"] = handle
         constructed[_C5_PREBUILT_ESTIMATOR_KEY] = estimator
     return constructed