[AZ-355] C4 PoseEstimator Protocol + factory + DTOs + composition

Land the foundational C4 surface AZ-358 (Marginals) and AZ-361
(Hybrid) build on top of:

- PoseEstimator Protocol (@runtime_checkable): estimate(...) +
  current_covariance_mode().
- Error hierarchy: PoseEstimatorError, PnpFailureError,
  PoseEstimatorConfigError; CovarianceDegradedWarning as a Warning
  subclass (warnings.warn path, not raised).
- ISam2GraphHandle Protocol stub (READ-ONLY view, get_pose_key only)
  decoupled from C5's concrete ISam2GraphHandleImpl.
- C4PoseConfig (frozen dataclass) + register on c4_pose import.
- runtime_root/pose_factory.build_pose_estimator with lazy-import
  fallback; INFO log c4.pose.strategy_loaded; shares ingest-thread
  binding with C5 per ADR-003.

DTO restructuring (cross-cutting): retire the legacy raw-4x4
PoseEstimate(int frame_id, datetime timestamp, pose_se3, ...) and
ship the contract shape PoseEstimate(UUID, LatLonAlt, Quat,
np.ndarray, CovarianceMode, PoseSourceLabel,
last_satellite_anchor_age_ms, emitted_at). C5 add_pose_anchor in
both gtsam_isam2 + eskf_baseline migrated in lockstep via
WGS84->ENU + Quat->R helpers; test fixtures updated. VIO output
stays on the raw shape until AZ-331 (C1 protocol) lands.

LatLonAlt upgraded to slots=True per AC-2. ThermalState stub added
to _types/thermal.py so the Protocol typechecks pre-AZ-302.

Tests: 25 new in tests/unit/c4_pose/test_az355_pose_protocol.py
covering AC-1..AC-10 + factory wiring + config validation; full
repo: 685 passed, 2 pre-existing CI-only skips.

Jira transition deferred: MCP "Not connected"; leftover entry in
_docs/_process_leftovers/2026-05-11_jira_transition_az355_deferred.md.

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

src/gps_denied_onboard/components/c4_pose/config.py

@@ -0,0 +1,64 @@
+"""C4 pose-estimator config block (AZ-355).
+
+Registered into the global config registry via
+``register_component_block("c4_pose", C4PoseConfig)`` on import of
+``gps_denied_onboard.components.c4_pose``. The runtime root reads
+``config.components["c4_pose"]`` and dispatches to the
+``opencv_gtsam`` strategy (the only one defined; the Protocol exists
+for ADR-009).
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Final
+
+from gps_denied_onboard.config.schema import ConfigError
+
+__all__ = ["KNOWN_POSE_STRATEGIES", "C4PoseConfig"]
+
+
+KNOWN_POSE_STRATEGIES: Final[frozenset[str]] = frozenset({"opencv_gtsam"})
+
+
+@dataclass(frozen=True)
+class C4PoseConfig:
+    """C4 pose-estimator config block.
+
+    Fields per the C4 contract §"Config-load-time validation":
+
+    * ``strategy`` — selects the concrete estimator. Currently only
+      ``"opencv_gtsam"`` is defined.
+    * ``ransac_iterations`` — OpenCV ``solvePnPRansac`` iteration
+      budget. Default 200 per the contract.
+    * ``ransac_reprojection_threshold_px`` — RANSAC inlier-distance
+      threshold. Default 4.0 pixels per the contract.
+    * ``thermal_throttle_threshold_celsius`` — informational only;
+      the actual ``ThermalState.throttle`` decision is owned by C7
+      (AZ-302). Default 75.0 °C.
+    """
+
+    strategy: str = "opencv_gtsam"
+    ransac_iterations: int = 200
+    ransac_reprojection_threshold_px: float = 4.0
+    thermal_throttle_threshold_celsius: float = 75.0
+
+    def __post_init__(self) -> None:
+        if self.strategy not in KNOWN_POSE_STRATEGIES:
+            raise ConfigError(
+                f"C4PoseConfig.strategy={self.strategy!r} not in {sorted(KNOWN_POSE_STRATEGIES)}"
+            )
+        if self.ransac_iterations <= 0:
+            raise ConfigError(
+                f"C4PoseConfig.ransac_iterations must be > 0; got {self.ransac_iterations}"
+            )
+        if self.ransac_reprojection_threshold_px <= 0.0:
+            raise ConfigError(
+                "C4PoseConfig.ransac_reprojection_threshold_px must be > 0; "
+                f"got {self.ransac_reprojection_threshold_px}"
+            )
+        if self.thermal_throttle_threshold_celsius <= 0.0:
+            raise ConfigError(
+                "C4PoseConfig.thermal_throttle_threshold_celsius must be > 0; "
+                f"got {self.thermal_throttle_threshold_celsius}"
+            )