refactor(spec): move observability seam into the spec layer (leanEthereum#830)

The spec layer's last upward dependency on node was the observability
import in fork_choice.py and state_transition.py. A prior refactor moved
the Interval type and protocol constants into the spec layer but left
this edge, so spec still reached up into node.

Relocate node/observability/ to spec/observability/. The package is
purely the vendor-neutral observer seam: a SpecObserver Protocol, a
no-op default, the observe_* context-manager timers, and set_observer.
Spec emits through it with a no-op default, so spec imports stay
side-effect-free. The concrete Prometheus backend stays in node/metrics
and the startup set_observer wiring stays in the CLI; only the seam moves.

Pure relocation, no behavior change. The test moves to the mirrored
spec path per the test-structure convention.

After this, grep -rn "lean_spec.node" src/lean_spec/spec/ is empty: the
spec layer no longer depends on node in either direction.

Alternative considered: leave the seam in node and inject an observer
into every spec entry point. Rejected as strictly worse churn that
fights the existing module-level singleton design; the seam is itself a
spec-level concern (the spec defines what it observes), so it belongs in
spec with the no-op default.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: alexanderlhicks

src/lean_spec/cli/run.py

@@ -22,9 +22,9 @@
 from lean_spec.node.networking.client import LiveNetworkEventSource
 from lean_spec.node.networking.gossipsub import GossipTopic
 from lean_spec.node.node import Node, NodeConfig
-from lean_spec.node.observability import set_observer
 from lean_spec.spec.forks import SubnetId
 from lean_spec.spec.forks.lstar.config import ATTESTATION_COMMITTEE_COUNT
+from lean_spec.spec.observability import set_observer
 
 logger = logging.getLogger(__name__)
 

src/lean_spec/spec/forks/lstar/fork_choice.py

@@ -3,10 +3,6 @@
 import math
 from collections import defaultdict
 
-from lean_spec.node.observability import (
-    observe_on_attestation,
-    observe_on_block,
-)
 from lean_spec.spec.crypto.merkleization import hash_tree_root
 from lean_spec.spec.crypto.xmss.interface import TARGET_SIGNATURE_SCHEME
 from lean_spec.spec.forks.lstar._base import LstarSpecBase, LstarStore
@@ -30,6 +26,10 @@
     ValidatorIndex,
 )
 from lean_spec.spec.forks.protocol import SpecBlockType, SpecStateType
+from lean_spec.spec.observability import (
+    observe_on_attestation,
+    observe_on_block,
+)
 from lean_spec.spec.ssz import Bytes32, Uint64
 
 

src/lean_spec/spec/forks/lstar/state_transition.py

@@ -4,9 +4,6 @@
 from collections.abc import Iterable, Sequence
 from typing import Any
 
-from lean_spec.node.observability import (
-    observe_state_transition,
-)
 from lean_spec.spec.crypto.merkleization import hash_tree_root
 from lean_spec.spec.forks.lstar._base import LstarSpecBase
 from lean_spec.spec.forks.lstar.containers import (
@@ -24,6 +21,9 @@
     Validators,
 )
 from lean_spec.spec.forks.protocol import SpecStateType
+from lean_spec.spec.observability import (
+    observe_state_transition,
+)
 from lean_spec.spec.ssz import ZERO_HASH, Boolean, Bytes32, SSZList, Uint64
 
 

…lean_spec/node/observability/__init__.py …lean_spec/spec/observability/__init__.pysrc/lean_spec/node/observability/__init__.py renamed to src/lean_spec/spec/observability/__init__.py src/lean_spec/node/observability/__init__.py renamed to src/lean_spec/spec/observability/__init__.py

@@ -11,7 +11,7 @@
 their own.
 """
 
-from lean_spec.node.observability.observer import (
+from lean_spec.spec.observability.observer import (
     SpecObserver,
     observe_on_attestation,
     observe_on_block,

…lean_spec/node/observability/observer.py …lean_spec/spec/observability/observer.pysrc/lean_spec/node/observability/observer.py renamed to src/lean_spec/spec/observability/observer.py src/lean_spec/node/observability/observer.py renamed to src/lean_spec/spec/observability/observer.py

@@ -10,14 +10,14 @@
 from prometheus_client import CollectorRegistry, Histogram
 
 from lean_spec.node.metrics import PrometheusObserver, registry as metrics
-from lean_spec.node.observability import (
+from lean_spec.spec.observability import (
     observe_on_attestation,
     observe_on_block,
     observe_state_transition,
     observer as observer_module,
     set_observer,
 )
-from lean_spec.node.observability.observer import _NullObserver
+from lean_spec.spec.observability.observer import _NullObserver
 
 
 @pytest.fixture