[AZ-266] [AZ-269] [AZ-277] [AZ-280] Cross-cutting log/config + SE3/SHA256 helpers

AZ-266: schema-compliant JSON logging entrypoint, level normalisation,
handler-topology guard, format-error fallback (log_record_schema v1.0.0).
AZ-269: env > YAML > defaults config loader, frozen Config dataclass,
missing-var fail-fast with pointer to .env.example, component-block registry.
AZ-277: GTSAM-backed SE3Utils (matrix<->SE3 + exp/log/adjoint) with strict
orthogonality, dtype, and bottom-row contract enforcement.
AZ-280: atomicwrites-backed write_atomic + independent verify +
order-deterministic aggregate_hash; sidecar format strictness.
pyproject.toml pins gtsam>=4.2,<5.0 and atomicwrites>=1.4,<2.0
(named-backend deps per the AZ-277 / AZ-280 contracts).
139 unit tests pass (44 new). Review verdict: PASS_WITH_WARNINGS;
findings are perf-NFR + journald deferrals, no blocking issues.

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

src/gps_denied_onboard/config/loader.py

@@ -1,16 +1,178 @@
-"""Config loader — STUB.
+"""`load_config` — the single entrypoint that materialises `Config` at startup.
 
-Concrete YAML + env-var loader is owned by AZ-269. Bootstrap exposes the load
-function as `NotImplementedError` so callers fail loudly until AZ-269 lands.
+Implements the `composition_root_protocol` contract v1.0.0 (E-CC-CONF /
+AZ-269 / AZ-246). Precedence (highest -> lowest):
+
+1. Environment variables (``env`` argument).
+2. YAML files (``paths``), in order — later paths override earlier ones.
+3. Documented defaults baked into the cross-cutting dataclasses.
+
+The returned `Config` is frozen end-to-end. Required env vars that fail
+to resolve raise `RequiredFieldMissingError` with the name of the
+offending variable and a pointer at ``.env.example``.
 """
 
 from __future__ import annotations
 
+from collections.abc import Mapping, Sequence
 from pathlib import Path
+from typing import Any, Final
 
-from gps_denied_onboard.config.schema import RuntimeConfig
+import yaml
+
+from gps_denied_onboard.config.schema import (
+    _COMPONENT_REGISTRY,
+    Config,
+    FdrConfig,
+    LogConfig,
+    RequiredFieldMissingError,
+    RuntimeConfig,
+    _replace_block,
+    _resolve_component_blocks,
+)
+
+__all__ = ["ENV_KEY_MAP", "load_config"]
 
 
-def load_runtime_config(yaml_path: Path) -> RuntimeConfig:
-    """Load a `RuntimeConfig` from a YAML file + environment overlay."""
-    raise NotImplementedError("Config loader concrete impl is AZ-269 (E-CC-CONF)")
+# Env-var -> (block, field) mapping. The composition root reads env vars
+# through this table so the YAML path and the env path stay in sync.
+ENV_KEY_MAP: Final[dict[str, tuple[str, str]]] = {
+    # Cross-cutting blocks
+    "GPS_DENIED_FC_PROFILE": ("runtime", "fc_profile"),
+    "GPS_DENIED_TIER": ("runtime", "tier"),
+    "DB_URL": ("runtime", "db_url"),
+    "CAMERA_CALIBRATION_PATH": ("runtime", "camera_calibration_path"),
+    "INFERENCE_BACKEND": ("runtime", "inference_backend"),
+    "TILE_CACHE_PATH": ("runtime", "tile_cache_path"),
+    "LOG_LEVEL": ("log", "level"),
+    "LOG_TIER": ("log", "tier"),
+    "LOG_SINK": ("log", "sink"),
+    "FDR_PATH": ("fdr", "path"),
+    "FDR_QUEUE_SIZE": ("fdr", "queue_size"),
+}
+
+# Env vars that MUST resolve to a non-empty value before `load_config`
+# can return (per AZ-263 AC-8 + AZ-269 AC-6). Missing values trigger
+# `RequiredFieldMissingError` with the variable name in the message.
+_REQUIRED_ENV_VARS: Final[tuple[str, ...]] = (
+    "GPS_DENIED_FC_PROFILE",
+    "GPS_DENIED_TIER",
+    "DB_URL",
+    "CAMERA_CALIBRATION_PATH",
+    "LOG_LEVEL",
+    "LOG_SINK",
+    "INFERENCE_BACKEND",
+    "FDR_PATH",
+    "TILE_CACHE_PATH",
+)
+
+# Field-name -> python type. We coerce string env vars + raw YAML scalars
+# into the dataclass's declared types so `Config.runtime.tier` is always
+# `int` regardless of source.
+_FIELD_COERCIONS: Final[dict[str, type]] = {
+    "tier": int,
+    "queue_size": int,
+    "level": str,
+    "sink": str,
+    "path": str,
+    "fc_profile": str,
+    "db_url": str,
+    "camera_calibration_path": str,
+    "inference_backend": str,
+    "tile_cache_path": str,
+    "overrun_policy": str,
+}
+
+
+def _coerce_value(field_name: str, raw: Any) -> Any:
+    target_type = _FIELD_COERCIONS.get(field_name)
+    if target_type is None or isinstance(raw, target_type):
+        return raw
+    try:
+        return target_type(raw)
+    except (TypeError, ValueError) as exc:
+        raise RequiredFieldMissingError(
+            f"config field {field_name!r}: cannot coerce {raw!r} to {target_type.__name__} ({exc})"
+        ) from exc
+
+
+def _load_yaml_files(paths: Sequence[Path]) -> dict[str, dict[str, Any]]:
+    """Merge YAML files in order: later paths win for the same block + field."""
+    merged: dict[str, dict[str, Any]] = {}
+    for path in paths:
+        data = yaml.safe_load(path.read_text()) or {}
+        if not isinstance(data, dict):
+            raise RequiredFieldMissingError(
+                f"YAML at {path} must be a mapping at the top level; got {type(data).__name__}"
+            )
+        for block_name, block_value in data.items():
+            if not isinstance(block_value, dict):
+                continue
+            merged.setdefault(block_name, {}).update(block_value)
+    return merged
+
+
+def _apply_env_overrides(layered: dict[str, dict[str, Any]], env: Mapping[str, str]) -> None:
+    """Overlay env-var values on the per-block override dictionaries."""
+    for env_key, (block_name, field_name) in ENV_KEY_MAP.items():
+        if env_key not in env:
+            continue
+        layered.setdefault(block_name, {})[field_name] = env[env_key]
+
+
+def _check_required_env(env: Mapping[str, str]) -> None:
+    """AC-6 + AZ-263 AC-8: missing required vars fail fast with a pointer."""
+    missing = [name for name in _REQUIRED_ENV_VARS if not env.get(name)]
+    if missing:
+        raise RequiredFieldMissingError(
+            "Missing required environment variable(s): "
+            + ", ".join(missing)
+            + ". See `.env.example` for the documented set."
+        )
+
+
+def load_config(
+    env: Mapping[str, str],
+    paths: Sequence[Path] = (),
+    *,
+    require_env: bool = True,
+) -> Config:
+    """Build a frozen `Config` from env + YAML files + documented defaults.
+
+    Precedence: env > YAML > defaults. `paths` may be empty; missing keys
+    fall to the dataclass-declared defaults.
+    """
+    if require_env:
+        _check_required_env(env)
+
+    yaml_overrides = _load_yaml_files(paths) if paths else {}
+    _apply_env_overrides(yaml_overrides, env)
+
+    runtime_block = _replace_block(
+        RuntimeConfig(),
+        {k: _coerce_value(k, v) for k, v in yaml_overrides.get("runtime", {}).items()},
+    )
+    log_block = _replace_block(
+        LogConfig(),
+        {k: _coerce_value(k, v) for k, v in yaml_overrides.get("log", {}).items()},
+    )
+    fdr_block = _replace_block(
+        FdrConfig(),
+        {k: _coerce_value(k, v) for k, v in yaml_overrides.get("fdr", {}).items()},
+    )
+
+    component_blocks = _resolve_component_blocks()
+    for slug, dataclass_type in _COMPONENT_REGISTRY.items():
+        block_overrides = yaml_overrides.get(slug, {})
+        if block_overrides:
+            component_blocks[slug] = _replace_block(
+                dataclass_type(),
+                {k: _coerce_value(k, v) for k, v in block_overrides.items()},
+            )
+
+    return Config(
+        runtime=runtime_block,
+        log=log_block,
+        fdr=fdr_block,
+        components=component_blocks,
+    )